Housekeeping.

- Added some more notes to config.h.

- Added the ability to override some of the #defines around Grbl in
config.h, like planner buffer size, line buffer size, serial
send/receive buffers. Mainly to centralize the configurations to be
able to port to different microcontrollers later.

limits.h

@@ -21,8 +21,6 @@
 #ifndef limits_h
 #define limits_h 
 
-#define MICROSECONDS_PER_ACCELERATION_TICK  (1000000/ACCELERATION_TICKS_PER_SECOND)
-
 // initialize the limits module
 void limits_init();