Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -9,6 +9,8 @@ If you'd like to contribute, please read the following document:
 * [PSR-1][2]: PSR-1 are standard conventions rules we use as a base (conversion of old code still in progress)
 * [PSR-2][3]: PSR-2 are more detailed standard conventions rules we use as base (conversion of old code still in progress)
 
+We expect contributions to be sent through Pull Requests, a special feature of Github. We recommend you follow this guide to understand a little more about the way it works: https://guides.github.com/activities/contributing-to-open-source/
+
 [1]: https://support.chamilo.org/projects/chamilo-18/wiki/Coding_conventions
 [2]: https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-1-basic-coding-standard.md
 [3]: https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md