Update release checklist with more information, more commands, and more correct flow (test, upload to test.docker.io, test, upload to get.docker.io, THEN tag&merge)

Docker-DCO-1.0-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3446 from tianon/update-checklist

Update release checklist with more information, more commands, and more ...