Updates for 2.2.0~alpha release

[rjbou]

• Update CHANGES
  • backport 2.1.x changelogs
  • Add master changelog
  • empty master_changes
• Bump version to 2.2.0~alpha
• Add @dra27 & @kit-ty-kate as authors

[XantreDev]

Hyped

[XantreDev]

How try it out?

[rjbou]

If you want to try it before the release, you can try master from now:

$ git clone https://github.com/ocaml/opam
$ cd opam
$ make cold

here is a more complete process of testing a branch (that is be master here).

Otherwise, in the next days, the pre-release will be published, with as usual all binaries, opam packages, etc.

[XantreDev]

Thanks