message: Update RBE CPU Dockerfile with instructions on how to build/push.
diff: +# To push a new version, run:
  +# $ docker build -f Dockerfile.rbe.cpu \
  +# --tag "gcr.io/tensorflow-testing/nosla-ubuntu16.04" .
  +# $ docker push gcr.io/tensorflow-testing/nosla-ubuntu16.04
  +
  FROM launcher.gcr.io/google/rbe-ubuntu16-04:r327695
  LABEL maintainer="Yu Yi <yiyu@google.com>"
  