Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+Contributions are **welcome** and will be fully **credited**.
+
+We accept contributions via Pull Requests on [Github](https://github.com/thephpleague/oauth2-client).
+
+
+## Pull Requests
+
+- **[PSR-2 Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md)** - The easiest way to apply the conventions is to install [PHP Code Sniffer](http://pear.php.net/package/PHP_CodeSniffer).
+
+- **Add tests!** - Your patch won't be accepted if it doesn't have tests.
+
+- **Document any change in behaviour** - Make sure the README and any other relevant documentation are kept up-to-date.
+
+- **Consider our release cycle** - We try to follow SemVer. Randomly breaking public APIs is not an option.
+
+- **Create topic branches** - Don't ask us to pull from your master branch.
+
+- **One pull request per feature** - If you want to do more than one thing, send multiple pull requests.
+
+- **Send coherent history** - Make sure each individual commit in your pull request is meaningful. If you had to make multiple intermediate commits while developing, please squash them before submitting.
+
+
+## Running Tests
+
+``` bash
+$ phpunit
+```
+
+
+**Happy coding**!