update compatibility matrix in README

Author: andydoro

README.md

@@ -38,6 +38,7 @@ ATSAM21D           |      X       |             |            |
 ATtiny85 @ 16MHz   |             |             |     X       | 
 ATtiny85 @ 8MHz    |             |             |     X       | 
 Intel Curie @ 32MHz |      X       |             |            | 
+STM32F2            |             |             |     X       | 
 
   * ATmega328 @ 16MHz : Arduino UNO, Adafruit Pro Trinket 5V, Adafruit Metro 328, Adafruit Metro Mini
   * ATmega328 @ 12MHz : Adafruit Pro Trinket 3V