modify readme

Author: facchinm

README.adoc

@@ -1,6 +1,6 @@
 = MIDIUSB Library for Arduino =
 
-This library allows an Arduino board with USB capabilites to act as a MIDI instrument over USB.
+This library allows an Arduino board with USB capabilites to act as a MIDI instrument over USB. The library is based on PluggableUSB, so is only compatible with IDE 1.6.6 and nightly builds (AVR core > 1.6.8)
 
 For more information about this library please visit us at
 http://www.arduino.cc/en/Reference/MIDIUSB