add hotfix help

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Update RELEASE-CHECKLIST with a few minor nits (periods, whitespace)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #9 from tianon/vieux-hotfix-minor-nits

Update RELEASE-CHECKLIST with a few minor nits (periods, whitespace)
Merge pull request #6916 from vieux/update_release_checklist_hotfix

Add hotfix help to the release checklist