Add contributing file

CONTRIBUTING.md

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+Contributing
+============
+
+First of all, **thank you** for contributing, **you are awesome**!
+
+Here are a few rules to follow in order to ease code reviews, and discussions before
+maintainers accept and merge your work.
+
+You MUST follow the [PSR-1](http://www.php-fig.org/psr/1/) and
+[PSR-2](http://www.php-fig.org/psr/2/). If you don't know about any of them, you
+should really read the recommendations. Can't wait? Use the [PHP-CS-Fixer
+tool](http://cs.sensiolabs.org/).
+
+You MUST run the test suite.
+
+You MUST write (or update) unit tests.
+
+You SHOULD write documentation.
+
+Please, write [commit messages that make
+sense](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html),
+and [rebase your branch](http://git-scm.com/book/en/Git-Branching-Rebasing)
+before submitting your Pull Request.
+
+One may ask you to [squash your
+commits](http://gitready.com/advanced/2009/02/10/squashing-commits-with-rebase.html)
+too. This is used to "clean" your Pull Request before merging it (we don't want
+commits such as `fix tests`, `fix 2`, `fix 3`, etc.).
+
+Also, while creating your Pull Request on GitHub, you MUST write a description
+which gives the context and/or explains why you are creating it.
+
+Thank you!