Add a note about ATmega32U4 incompatibility to SerialEvent example

serialEvent() does not work on the ATmega32U4 boards.

Reference: https://github.com/arduino/Arduino/issues/1031

Author: per1234

Diff for: build/shared/examples/04.Communication/SerialEvent/SerialEvent.ino

@@ -8,6 +8,9 @@
  A good test for this is to try it with a GPS receiver
  that sends out NMEA 0183 sentences.
 
+ NOTE: The serialEvent() feature is not available on the
+ Leonardo, Micro, or other ATmega32U4 based boards.
+ 
  Created 9 May 2011
  by Tom Igoe