Minor copy edits and updates to README. 

Also added a CTA for keeping the projects list current.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
@vieux I thought about it, then decided not to since they could easily be reading the ReadME from a local clone of the repo and would thus have the NOTICE right there. Could easily be persuaded to add a link, though, if you think it necessary.