Update README.md

Author: TheKitty

Monster_M4sk_Watching_You/README.md

@@ -4,6 +4,13 @@ Make a person responsive set of eyes.
 
 Follow the Adafruit learn guide here: https://learn.adafruit.com/monster-m4sk-is-watching-you/
 
+The source code is the same as the [M4_Eyes](https://github.com/adafruit/Adafruit_Learning_System_Guides/tree/master/M4_Eyes) code
+with the following changes:
+
+In **user.cpp**, change the first line from #if 1 to #if 0. In **user_watch.cpp**, do the opposite: change the first line from #if 0 to #if 1.
+
+This ensures that exactly one version of the **user_setup()** and **user_loop()** functions is defined.
+
 Adafruit invests time and resources providing this open source code,
 please support Adafruit and open-source hardware by purchasing
 products from [Adafruit](https://www.adafruit.com)!