Updated License in readme file to MIT to match the rest of of the project.
Updated License in readme file to MIT to match the rest of of the project.

PiperOrigin-RevId: 147882366

FirebaseExtended/cocos2dx-cpp-sample,FirebaseExtended/cocos2dx-cpp-sample,FirebaseExtended/cocos2dx-cpp-sample,FirebaseExtended/cocos2dx-cpp-sample