RELEASING.md: Document command to start making release notes
RELEASING.md: Document command to start making release notes

It's fine if the command changes in the future, but we do want to start making
the release notes earlier than later.

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