minor #10373 added the BC docs to the contributing file (fabpot)

This PR was merged into the 2.5-dev branch.

Discussion
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added the BC docs to the contributing file

Commits
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2f9432a added the BC docs to the contributing file

CONTRIBUTING.md

@@ -1,11 +1,20 @@
 Contributing
 ------------
 
-Symfony2 is an open source, community-driven project. 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].
+Symfony2 is an open source, community-driven project.
+
+If you'd like to contribute, please read the following documents:
+
+* [Contributing Code][1]: The document index related to contributions;
+
+* [Submitting a Patch][2]: Guidelines for submitting a pull request;
+
+* [Pull Request Template][3]: Template header to use in your pull request
+  description;
+
+* [Backwards Compatibility][4]: Backward compatibility rules.
 
 [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
+[4]: http://symfony.com/doc/current/contributing/code/bc.html#working-on-symfony-code