[atomic] Using Fedora updates-testing with Fedora Atomic 22

[Colin Walters <walters verbum org>]

Those of you who have used Fedora for a while will likely be familiar with the
concept of the "updates-testing" repository.  I've now documented the fact
that there's an OSTree branch of Fedora Atomic 22 that tracks it:

https://fedoraproject.org/wiki/QA:Updates_Testing#Using_it_with_Fedora_Atomic_22

Once you run this command, your system will track updates-testing.  You can
then reboot and use the new system to provide karma for updates to the
individual packages in Atomic, such as:

https://admin.fedoraproject.org/updates/FEDORA-2015-8977/kubernetes-0.17.1-2.fc22
and
https://admin.fedoraproject.org/updates/FEDORA-2015-9288/ostree-2015.6-2.fc22

(To pick two topical ones, but currently there are many other updates queued
 in updates-testing, `kernel`, `less`, `openssh`, etc.)