Configure Git contribution guidelines for github.com

Many open source projects use github.com for their contribution process.
Although we mirror the Git core repository to github.com [1] we do not
use any other github.com service. This is unknown/unexpected to a
number of (potential) contributors and consequently they create Pull
Requests against our mirror with their contributions. These Pull
Requests become stale. This is frustrating to them as they think we
ignore them and it is also unsatisfactory for us as we miss potential
code improvements and/or new contributors.

GitHub contribution guidelines and a GitHub Pull Request template that
is visible to every Pull Request creator can be configured with special
files in a Git repository [2]. Let's make use of this!

[1] https://github.com/git/git
[2] https://help.github.com/articles/creating-a-pull-request-template-for-your-repository/Signed-off-by: NLars Schneider <larsxschneider@gmail.com>
Helped-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

.github/CONTRIBUTING.md

+## Contributing to Git
+
+Thanks for taking the time to contribute to Git! Please be advised that the
+Git community does not use github.com for their contributions. Instead, we use
+a mailing list (git@vger.kernel.org) for code submissions, code
+reviews, and bug reports.
+
+Nevertheless, you can use [submitGit](http://submitgit.herokuapp.com/) to
+conveniently send your Pull Requests commits to our mailing list.
+
+Please read ["A note from the maintainer"](https://git.kernel.org/pub/scm/git/git.git/plain/MaintNotes?h=todo)
+to learn how the Git project is managed, and how you can work with it.
+In addition, we highly recommend you to read [our submission guidelines](../Documentation/SubmittingPatches).
+
+If you prefer video, then [this talk](https://www.youtube.com/watch?v=Q7i_qQW__q4&feature=youtu.be&t=6m4s)
+might be useful to you as the presenter walks you through the contribution
+process by example.
+
+Your friendly Git community!

.github/PULL_REQUEST_TEMPLATE.md

+Thanks for taking the time to contribute to Git! Please be advised that the
+Git community does not use github.com for their contributions. Instead, we use
+a mailing list (git@vger.kernel.org) for code submissions, code reviews, and
+bug reports. Nevertheless, you can use submitGit to conveniently send your Pull
+Requests commits to our mailing list.
+
+Please read the "guidelines for contributing" linked above!