Update the README file
Update the README file

Add the --no-promote option to the deploy command, and remove info on
local Docker builds.

Add commands to tag the deployed version.

R=whesse@google.com
BUG=

Review URL: https://codereview.chromium.org/1839973005 .

dart-lang/pub-dev,dart-lang/pub-dev,dart-lang/pub-dev,dart-lang/pub-dev