Add comments about EEPROM size config

teensy3/avr/eeprom.h

  * THE SOFTWARE.
  */
 
+// Guidelines for editing this file:
+// https://forum.pjrc.com/threads/34537-Teensy-LC-Increase-EEPROM-Size/page2
+
 #ifndef _AVR_EEPROM_H_
 #define _AVR_EEPROM_H_ 1
 

teensy3/eeprom.c

  * SOFTWARE.
  */
 
+// To configure the EEPROM size, edit E2END in avr/eeprom.h.
+//
+// Do *NOT* edit EEPROM_SIZE in this file.  It will automatically
+// change based on your changes to E2END in avr/eeprom.h.
+//
+// Generally you should avoid editing this code, unless you really
+// know what you're doing.
+
 #include "kinetis.h"
 #include <avr/eeprom.h>
 //#include "HardwareSerial.h"