README.md: Document protobuf-gradle-plugin config for lite
README.md: Document protobuf-gradle-plugin config for lite

This is part of the examples and other documentation, but a user
starting with the README would find things not working and it be very
unclear why.

Realized this was an issue because of
https://stackoverflow.com/q/66028045/4690866 .

dapengzhang0/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,grpc/grpc-java,ejona86/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java