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.

serial.h

@@ -27,8 +27,13 @@
 
 #include "nuts_bolts.h"
 
-#define RX_BUFFER_SIZE 128
-#define TX_BUFFER_SIZE 64
+#ifndef RX_BUFFER_SIZE
+  #define RX_BUFFER_SIZE 128
+#endif
+#ifndef TX_BUFFER_SIZE
+  #define TX_BUFFER_SIZE 64
+#endif
+
 #define SERIAL_NO_DATA 0xff
 
 #ifdef ENABLE_XONXOFF