Configure Git contribution guidelines for github.com

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: Lars Schneider <larsxschneider@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>