Added pointer for ESP8266 fork

As requested in https://github.com/z3t0/Arduino-IRremote/issues/400

Author: marcmerlin

README.md

@@ -26,6 +26,7 @@ Tutorials and more information will be made available on [the official homepage]
 - ATmega64, 128
 - ATtiny 84 / 85
 - ESP32 (receive only)
+- ESP8266 is supported in a fork based on an old codebase that isn't as recent, but it works reasonably well given that perfectly timed sub millisecond interrupts are different on that chip. See https://github.com/markszabo/IRremoteESP8266
 
 We are open to suggestions for adding support to new boards, however we highly recommend you contact your supplier first and ask them to provide support from their side.