Merge pull request #871 from whosawhatsis/fwretract

Update Readme descriptions for M200, M207 and M208

README.md

 *  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 - Set filament diameter
+*  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/sec] Z[additional zlift/hop]
-*  M208 - set recover=unretract length S[positive mm surplus to the M207 S*] F[feedrate mm/sec]
+*  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