update compatibility matrix in README

Author: andydoro

README.md

@@ -28,7 +28,8 @@ Atmega328 @ 16MHz |      X       |             |            |
 Atmega328 @ 12MHz |             |      X       |            | 
 Atmega32u4 @ 16MHz |      X       |             |            | 
 Atmega32u4 @ 8MHz |             |      X       |            | 
-ESP8266           |             |             |     X       | 
+ESP8266           |             |      X       |            |  In function 'void setup()':
+servo.pde:42:18: warning: 'void TwoWire::pins(int, int)' is deprecated (declared at /Users/james/Library/Arduino15/packages/esp8266/hardware/esp8266/2.0.0/libraries/Wire/Wire.h:54) [-Wdeprecated-declarations]
 Atmega2560 @ 16MHz |             |             |     X       | 
 ATSAM3X8E         |             |             |     X       | 
 ATSAM21D          |             |             |     X       |