Add CONTRIBUTING.md file to help contributors know how to contribute

CONTRIBUTING.md

@@ -0,0 +1,14 @@
+Contributing
+------------
+
+Chamilo LMS is a free software, community-driven project.
+
+If you'd like to contribute, please read the following document:
+
+* [Coding conventions][1]: The main conventions 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)
+
+[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