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.

planner.h

@@ -23,7 +23,9 @@
 #define planner_h
                  
 // The number of linear motions that can be in the plan at any give time
-#define BLOCK_BUFFER_SIZE 18
+#ifndef BLOCK_BUFFER_SIZE
+  #define BLOCK_BUFFER_SIZE 18
+#endif
 
 // This struct is used when buffering the setup for each linear movement "nominal" values are as specified in 
 // the source g-code and may never actually be reached if acceleration management is active.