doc(README): add basic contribution guidelines

The main point is that from now on, tests are required to add
new features. If the fix is minor enough, not having a test
is probably alright.

That distinction is not represented in the contribution guide
as more tests are better - people should prefer to have a test
whenever they contribute anything.

My motivation to finally do this is the sad realization that
I grow too unconfident about the quality of some contributions
without having tests that proof they are valid. It's not enough
to not break anything that exists, as the current test-suite is
certainly not perfect either.

Author: Byron

CONTRIBUTING.md

@@ -0,0 +1,6 @@
+### How to contribute
+
+* [fork this project](https://github.com/gitpython-developers/GitPython/fork) on github
+* Add yourself to AUTHORS.md and write your patch. **Write a test that fails unless your patch is present.**
+* Initiate a pull request
+

README.md

@@ -45,11 +45,9 @@ Then run:
 
     tox
 
-### SOURCE
+### Contributions
 
-GitPython's git repo is available on GitHub, which can be browsed at [github](https://github.com/gitpython-developers/GitPython) and cloned like that:
-
-    git clone https://github.com/gitpython-developers/GitPython
+Please have a look at the [contributions file][contributing].
 
 ### Live Coding
 
@@ -100,3 +98,4 @@ Now that there seems to be a massive user base, this should be motivation enough
 
 [twitch-channel]: http://www.twitch.tv/byronimo/profile
 [youtube-playlist]: https://www.youtube.com/playlist?list=PLMHbQxe1e9MnoEcLhn6Yhv5KAvpWkJbL0
+[contributing]: https://github.com/gitpython-developers/GitPython/blob/master/README.md