[Dune] Dune 2.2 release branches

[Oliver Sander]

Dear Dune,
I have just created the release branches for the upcoming 2.2 of the Dune
core modules. Instructions on how to obtain these branches are given on our
download page, http://dune-project.org/download.html. Please go and test
the release branches, and report the problems that you encounter.

@Ansgar: you probably need release candidate tarballs?  I'll build you some
tomorrow.

enjoy,
Oliver