Add Proguard / R8 rules for generated code

Hello,

When deploying the generated code in release on Android, the app crashes immediately because the gRPC generated code cannot actually be obfuscated to match the proto protocol I assume.

Please add proguard / R8 rules that apply to the generated code (even if you need to add it into a package to do so), and/or add a note into the README.md of the project about this. 

Thank you! 