Add deprecation warning in README

[felipemsantana]

Since RN 0.29 there's an official module (in NativeModules.AndroidPermissions) to request permissions in Android Marshmallow, although it is undocumented.

Will be released soon a new documented lib (facebook/react-native#9292).

This module should still be useful to who is using RN <= 0.28 and can't upgrade.

[lucasferreira]

Thanks for the warning @FMatosS

But we will wait for the update in oficial documentation until deprecate this module ;)