Update README with Unity installation instructions

Author: Adrien Couque

README.md

@@ -2,3 +2,6 @@
 
 This is a sample project built for the reverse engineering of the Android version of Pokémon Go.
 For more information, read here: https://applidium.com/en/news/unbundling_pokemon_go/.
+
+If you want to build this project, you will need to get `classes.jar`, that is provided by Unity.
+See in android/lunarconsole/build.gradle how to link to a Unity installation on Mac.