README, CHANGES: .txt -> .rst
So that both readme and changelog automatically render well on
https://lab.nexedi.com/nexedi/cloudooo and on
https://github.com/Nexedi/cloudooo
CHANGES is also renamed -> CHANGELOG along the way, because that's what gitlab autodetects as changelog file name:
https://lab.nexedi.com/nexedi/gitlab-ce/blob/827d3914/app/models/repository.rb#L353