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 stall [2]. 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 offers a way to notify Pull Request contributors about the
contribution guidelines for a project [3]. Let's make use of this!

[1] https://github.com/git/git
[2] https://github.com/git/git/pulls
[3] https://help.github.com/articles/creating-a-pull-request-template-for-your-repository/

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>

Author: larsxschneider

.github/CONTRIBUTING.md

@@ -0,0 +1,10 @@
+## 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](http://public-inbox.org/git/) for code submissions, code reviews, and bug reports.
+
+Please [read the maintainer notes](http://repo.or.cz/w/git.git?a=blob_plain;f=MaintNotes;hb=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 Git community

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,3 @@
+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 for code submissions, code reviews, and bug reports.
+
+Please read the "guidelines for contributing" linked above!