[xmonad] Announcing xmonad-testing: Help us test xmonad for v0.13

[Peter Jones]

We're working on the v0.13 release but we need your help. We'd like to
process as many pull requests/patches as possible before the release.
And of course, we'd like it to be as rock solid as possible.

We now have a new repo that should help you participate:

https://github.com/xmonad/xmonad-testing

Even if you don't know how to use `git` you can help. The `README.md`
file for the `xmonad-testing` repository contains instructions on
testing someone else's pull request. Just follow the steps!

If you have some spare cycles here's how you can help:

* Test someone's pull request using xmonad-testing

* Review open issues and see if you can fix something :)

Thanks everyone!