Added contribution rules.

Author: guillermo.fisher

CONTRIBUTING.md

@@ -0,0 +1,10 @@
+# Contributing
+
+## Pull Requests
+
+1. Create your own [fork](https://help.github.com/articles/fork-a-repo) of the repo.
+2. Send a pull request with your changes.
+
+## Style Guide
+
+All pull requests must adhere to the [PSR-2 standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md).