Create CONTRIBUTING.md

.github/CONTRIBUTING.md

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+# CONTRIBUTION GUIDELINES
+
+Contributions are **welcome** and will be fully **credited**.
+
+We accept contributions via pull requests on Github. Please review these guidelines before continuing.
+
+## Guidelines
+
+* Please follow the [PSR-2 Coding Style Guide](https://www.php-fig.org/psr/psr-2/), enforced by [StyleCI](https://styleci.io/).
+* Ensure that the current tests pass, and if you've added something new, add the tests where relevant.
+* Send a coherent commit history, making sure each individual commit in your pull request is meaningful.
+* You may need to [rebase](https://git-scm.com/book/en/v2/Git-Branching-Rebasing) to avoid merge conflicts.
+* If you are changing or adding to the behaviour or public api, you may need to update the docs.
+* Please remember that we follow [SemVer](https://semver.org/).
+
+## Running Tests
+
+First, install the dependencies using [Composer](https://getcomposer.org/):
+
+```bash
+$ composer install
+```
+
+Then run [PHPUnit](https://phpunit.de/):
+
+```bash
+$ vendor/bin/phpunit
+```
+
+* The tests will be automatically run by [Travis CI](https://travis-ci.org/) against pull requests.
+* We also have [StyleCI](https://styleci.io/) setup to automatically fix any code style issues.