GitLab

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

/cc @kazuhiko, @gabriel

/reviewed-on nexedi/cloudooo!3