adding contributing file

Author: dbu

CONTRIBUTING.md

@@ -0,0 +1,12 @@
+Contributing
+------------
+
+Symfony2 CMF is an open source, community-driven project. We follow the same
+guidelines as core Symfony2. If you'd like to contribute, please read the
+[Contributing Code][1] part of the documentation. If you're submitting a pull
+request, please follow the guidelines in the [Submitting a Patch][2] section
+and use the [Pull Request Template][3].
+
+[1]: http://symfony.com/doc/current/contributing/code/index.html
+[2]: http://symfony.com/doc/current/contributing/code/patches.html#check-list
+[3]: http://symfony.com/doc/current/contributing/code/patches.html#make-a-pull-request