[README] Adding a small section with information about contributing.

README.md

@@ -30,6 +30,15 @@ like us, you think that Symfony2 can help speed up your development and take
 the quality of your work to the next level, read the official
 [Symfony2 documentation][3].
 
+Contributing
+------------
+
+Symfony2 is an open source, community-driven project. If you'd like to contribute,
+please read the [Contributing Code][4] part of the documentation. If you're submitting
+a pull request, please follow the guidelines in the [Submitting a Patch][5] section.
+
 [1]: http://symfony.com/download
 [2]: http://symfony.com/get_started
 [3]: http://symfony.com/doc/current/
+[4]: http://symfony.com/doc/current/contributing/code/index.html
+[5]: http://symfony.com/doc/current/contributing/code/patches.html#check-list