Added M112

Marlin/Marlin_main.cpp

 #endif
       setWatch();
       break;
+    case 112: //  M112 -Emergency Stop
+      kill();
+      break;
     case 140: // M140 set bed temp
       if (code_seen('S')) setTargetBed(code_value());
       break;

README.md

 *  M107 - Fan off
 *  M109 - Sxxx Wait for extruder current temp to reach target temp. Waits only when heating
 *         Rxxx Wait for extruder current temp to reach target temp. Waits when heating and cooling
+*  M112 - Emergency stop
 *  M114 - Output current position to serial port
 *  M115 - Capabilities string
 *  M117 - display message