Update README.md

README.md

 
 http://www.pjrc.com/teensy/td_download.html
 
+This version implements overclock to 120MHz, 144MHz and 168MHz.
+
+Howto:
+1. Overwrite Arduino\hardware\teensy\boards.txt with this version 
+2. Overwrite all in core/teensy3 with this version
+
+