update compatibility matrix in README

Author: andydoro

README.md

@@ -28,7 +28,7 @@ Place the Adafruit_LSM9DS0 library folder your *arduinosketchfolder*/libraries/
 MCU                | Tested Works | Doesn't Work | Not Tested  | Notes
 ------------------ | :----------: | :----------: | :---------: | -----
 Atmega328 @ 16MHz  |      X       |             |            | 
-Atmega328 @ 12MHz  |             |             |     X       | 
+Atmega328 @ 12MHz  |      X       |             |            | 
 Atmega32u4 @ 16MHz |             |             |     X       | 
 Atmega32u4 @ 8MHz  |             |             |     X       | 
 ESP8266            |             |             |     X       |