Move the contribution guidelines to the .github folder

Author: bbatsov

CONTRIBUTING.md .github/CONTRIBUTING.md

@@ -179,7 +179,7 @@ Patches under any form are always welcome! GitHub pull requests are even better!
 
 Before submitting a patch or a pull request make sure all tests are
 passing and that your patch is in line with the [contribution
-guidelines](CONTRIBUTING.md).
+guidelines](.github/CONTRIBUTING.md).
 
 ### Working with mranderson (inlining dependencies)