RELEASING.md: add links to auto release documentation (#4444)
RELEASING.md: add links to auto release documentation (#4444)

The steps are still valid, and should be considered the source of
truth. However, these steps have been automated as a kokoro job. Add a
link to the documentation for the kokoro job.
carl-mastrangelo/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,zhangkun83/grpc-java,ejona86/grpc-java,elandau/grpc-java,zhangkun83/grpc-java,carl-mastrangelo/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,carl-mastrangelo/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,elandau/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,zhangkun83/grpc-java,zhangkun83/grpc-java,elandau/grpc-java,carl-mastrangelo/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,ejona86/grpc-java,elandau/grpc-java,grpc/grpc-java,dapengzhang0/grpc-java,dapengzhang0/grpc-java