Added a small contributing note

Author: edorian

CONTRIBUTING.md

@@ -0,0 +1,5 @@
+Pull Requests for bug fixes should be made against the current release branch (1.2). 
+
+Pull Requests for new features should be made against master.
+
+For further notes please refer to [https://github.com/sebastianbergmann/phpunit/blob/master/CONTRIBUTING.md](https://github.com/sebastianbergmann/phpunit/blob/master/CONTRIBUTING.md)