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.

protocol.h

@@ -29,7 +29,9 @@
 // characters. In future versions, this will be increased, when we know how much extra
 // memory space we can invest into here or we re-write the g-code parser not to have his 
 // buffer.
-#define LINE_BUFFER_SIZE 50
+#ifndef LINE_BUFFER_SIZE
+  #define LINE_BUFFER_SIZE 50
+#endif
 
 // Initialize the serial protocol
 void protocol_init();