Added documentation to the README.md for the sled Z probe option. CAB

Marlin/Configuration.h

   #define Z_RAISE_BEFORE_PROBING 15    //How much the extruder will be raised before traveling to the first probing point.
   #define Z_RAISE_BETWEEN_PROBINGS 5  //How much the extruder will be raised when traveling from between next probing points
 
+  #define Z_PROBE_SLED // turn on if you have a z-probe mounted on a sled like those designed by Charles Bell
+  #define SLED_DOCKING_OFFSET 5 // the extra distance the X axis must travel to pickup the sled. 0 should be fine but you can push it further if you'd like.
 
   //If defined, the Probe servo will be turned on only during movement and then turned off to avoid jerk
   //The value is the delay to turn the servo off after powered on - depends on the servo speed; 300ms is good value, but you can try lower it.

README.md

 ==========================
 Marlin 3D Printer Firmware
 ==========================
-[![Coverity Scan Build Status](https://scan.coverity.com/projects/2224/badge.svg)](https://scan.coverity.com/projects/2224)
-
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+
 Marlin has a GPL license because I believe in open development.
 Please do not use this code in products (3D printers, CNC etc) that are closed source or are crippled by a patent.
 
 *  G28 - Home all Axis
 *  G29 - Detailed Z-Probe, probes the bed at 3 points.  You must de at the home position for this to work correctly.
 *  G30 - Single Z Probe, probes bed at current XY location.
+*  G31 - Dock Z Probe sled (if enabled)
+*  G32 - Undock Z Probe sled (if enabled)
 *  G90 - Use Absolute Coordinates
 *  G91 - Use Relative Coordinates
 *  G92 - Set current position to cordinates given
 *  M140 - Set bed target temp
 *  M190 - Sxxx Wait for bed current temp to reach target temp. Waits only when heating
 *         Rxxx Wait for bed current temp to reach target temp. Waits when heating and cooling
-*  M200 D<millimeters>- set filament diameter and set E axis units to cubic millimeters (use S0 to set back to millimeters).
+*  M200 D<millimeters>- set filament diameter and set E axis units to cubic millimeters (use S0 to set back to millimeters).
 *  M201 - Set max acceleration in units/s^2 for print moves (M201 X1000 Y1000)
 *  M202 - Set max acceleration in units/s^2 for travel moves (M202 X1000 Y1000) Unused in Marlin!!
 *  M203 - Set maximum feedrate that your machine can sustain (M203 X200 Y200 Z300 E10000) in mm/sec
 *  M204 - Set default acceleration: S normal moves T filament only moves (M204 S3000 T7000) im mm/sec^2  also sets minimum segment time in ms (B20000) to prevent buffer underruns and M20 minimum feedrate
 *  M205 -  advanced settings:  minimum travel speed S=while printing T=travel only,  B=minimum segment time X= maximum xy jerk, Z=maximum Z jerk, E=maximum E jerk
 *  M206 - set additional homeing offset
-*  M207 - set retract length S[positive mm] F[feedrate mm/min] Z[additional zlift/hop], stays in mm regardless of M200 setting
-*  M208 - set recover=unretract length S[positive mm surplus to the M207 S*] F[feedrate mm/min]
+*  M207 - set retract length S[positive mm] F[feedrate mm/min] Z[additional zlift/hop], stays in mm regardless of M200 setting
+*  M208 - set recover=unretract length S[positive mm surplus to the M207 S*] F[feedrate mm/min]
 *  M209 - S<1=true/0=false> enable automatic retract detect if the slicer did not support G10/11: every normal extrude-only move will be classified as retract depending on the direction.
 *  M218 - set hotend offset (in mm): T<extruder_number> X<offset_on_X> Y<offset_on_Y>
 *  M220 S<factor in percent>- set speed factor override percentage
 ===============================================
 Instructions for configuring Bed Auto Leveling
 ===============================================
+There are two options for this feature. You may choose to use a servo mounted on the X carriage or you may use a sled that mounts on the X axis and can be docked when not in use.
+See the section for each option below for specifics about installation and configuration. Also included are instructions that apply to both options.
+
+Note for RAMPS users:
+---------------------
+
+By default, RAMPS have no power on servo bus (if you happen to have a multimeter, check the voltage on servo power pins).
+In order to get the servo working, you need to supply 5V to 5V pin.. You can do it using your power supply (if it has a 5V output) or jumping the "Vcc" from Arduino to the 5V RAMPS rail.
+These 2 pins are located just between the Reset Button and the yellow fuses... There are marks in the board showing 5V and VCC.. just connect them..
+If jumping the arduino Vcc do RAMPS 5V rail, take care to not use a power hungry servo, otherwise you will cause a blackout in the arduino board ;-)
+
+Instructions for Both Options
+-----------------------------
+
 Uncomment the "ENABLE_AUTO_BED_LEVELING" define (commented by default)
 
+The following options define the probing positions. These are good starting values.
+I recommend to keep a better clearance from borders in the first run and then make the probes as close as possible to borders:
+
+* \#define LEFT_PROBE_BED_POSITION 30
+* \#define RIGHT_PROBE_BED_POSITION 140
+* \#define BACK_PROBE_BED_POSITION 140
+* \#define FRONT_PROBE_BED_POSITION 30
+
+A few more options:
+
+* \#define XY_TRAVEL_SPEED 6000
+
+X and Y axis travel speed between probes, in mm/min.
+Bear in mind that really fast moves may render step skipping. 6000 mm/min (100mm/s) is a good value.
+
+* \#define Z_RAISE_BEFORE_PROBING 10
+* \#define Z_RAISE_BETWEEN_PROBINGS 10
+
+The Z axis is lifted when traveling to the first probe point by Z_RAISE_BEFORE_PROBING value
+and then lifted when traveling from first to second and second to third point by Z_RAISE_BETWEEN_PROBINGS.
+All values are in mm as usual.
+
+Servo Option Notes
+------------------
 You will probably need a swivel Z-MIN endstop in the extruder. A rc servo do a great job.
 Check the system working here: http://www.youtube.com/watch?v=3IKMeOYz-1Q (Enable English subtitles)
 Teasing ;-) video: http://www.youtube.com/watch?v=x8eqSQNAyro
 
 * \#define SERVO_ENDSTOP_ANGLES {0,0, 0,0, 165,60} // X,Y,Z Axis Extend and Retract angles
 
-
 The first define tells firmware how many servos you have.
 The second tells what axis this servo will be attached to. In the example above, we have a servo in Z axis.
 The third one tells the angle in 2 situations: Probing (165º) and resting (60º). Check this with command M280 P0 S{angle} (example: M280 P0 S60 moves the servo to 60º)
 
-For RAMPS users:
-----------------
-
-By default, RAMPS have no power on servo bus (if you happen to have a multimeter, check the voltage on servo power pins).
-In order to get the servo working, you need to supply 5V to 5V pin.. You can do it using your power supply (if it has a 5V output) or jumping the "Vcc" from Arduino to the 5V RAMPS rail.
-These 2 pins are located just between the Reset Button and the yellow fuses... There are marks in the board showing 5V and VCC.. just connect them..
-If jumping the arduino Vcc do RAMPS 5V rail, take care to not use a power hungry servo, otherwise you will cause a blackout in the arduino board ;-)
-
-
 Next you need to define the Z endstop (probe) offset from hotend.
 My preferred method:
 
 * \#define Z_PROBE_OFFSET_FROM_EXTRUDER -5.1
 
 
-The following options define the probing positions. These are good starting values.
-I recommend to keep a better clearance from borders in the first run and then make the probes as close as possible to borders:
+Sled Option Notes
+-----------------
+The sled option uses an electromagnet to attach and detach to/from the X carriage. See http://www.thingiverse.com/thing:396692 for more details on how to print and install this feature. It uses the same connections as the servo option.
 
-* \#define LEFT_PROBE_BED_POSITION 30
-* \#define RIGHT_PROBE_BED_POSITION 140
-* \#define BACK_PROBE_BED_POSITION 140
-* \#define FRONT_PROBE_BED_POSITION 30
+To use the sled option, you must define two additional things in Configuration.h:
 
-A few more options:
+* \#define Z_PROBE_SLED
+* \#define SLED_DOCKING_OFFSET 5
 
-* \#define XY_TRAVEL_SPEED 6000
+Uncomment the Z_PROBE_SLED to define to enable the sled (commented out by default).
 
-X and Y axis travel speed between probes, in mm/min.
-Bear in mind that really fast moves may render step skipping. 6000 mm/min (100mm/s) is a good value.
+Uncomment the SLED_DOCKING_OFFSET to set the extra distance the X axis must travel to dock the sled. This value can be found by moving the X axis to its maximum position then measure the distance to the right X end and subtract the width of the sled (23mm if you printed the sled from Thingiverse).
 
-* \#define Z_RAISE_BEFORE_PROBING 10
-* \#define Z_RAISE_BETWEEN_PROBINGS 10
+Next you need to define the Z endstop (probe) offset from hotend.
+My preferred method:
 
-The Z axis is lifted when traveling to the first probe point by Z_RAISE_BEFORE_PROBING value
-and then lifted when traveling from first to second and second to third point by Z_RAISE_BETWEEN_PROBINGS.
-All values are in mm as usual. 
+* a) Home the X and Y axes.
+* b) Move the X axis to about the center of the print bed. Make a mark on the print bed.
+* c) Move the Y axis to the maximum position. Make another mark.
+* d) Home the X axis and use a straight edge to make a line between the two points.
+* e) Repeat (b)-(d) reversing the X and Y. When you are done you will have two lines on the print bed. We will use these to measure the offset for the Z probe endstop.
+* f) Move the nozzle so that it is positioned on the center point of the two lines. You can use fine movement of 0.1mm to get it as close as possible. Note the position of X and Y.
+* g) Zero the Z axis with the G92 Z0 command.
+* h) Raise the Z axis about 20mmm.
+* i) Use the G32 command to retrieve the sled.
+* j) Now more the X and Y axis to the position recorded in (f).
+* k) Lower the Z axis in 0.1mm steps until you hear the "click" meaning the mechanical endstop was trigged. You can confirm with the M119 command. Note the position of the Z axis.
+* l) Make a mark on the print bed where the endstop lever has touched the print bed. Raise the Z-axis about 30mm to give yourself some room.
+* m) Now measure the distance from the center point to the endstop impact site along the X and Y axis using the lines drawn previously.
+* n) Fill in the values below. If the endstop mark is in front of the line running left-to-right, use positive values. If it is behind, use negative values. For the Z axis use the value from (k) and subtract 0.1mm.
+
+For example, suppose you measured the endstop position and it was 20mm to the right of the line running front-to-back, 10mm toward the front of the line running left-to-right, and the value from (k) was 2.85. The values for the defines would be:
+
+* \#define X_PROBE_OFFSET_FROM_EXTRUDER 20
+* \#define Y_PROBE_OFFSET_FROM_EXTRUDER 10
+* \#define Z_PROBE_OFFSET_FROM_EXTRUDER 2.75
 
 That's it.. enjoy never having to calibrate your Z endstop neither leveling your bed by hand anymore ;-)