add contribution guide

CONTRIBUTING.md

@@ -0,0 +1,43 @@
+# Contributing
+
+Contributions are **welcome** and will be fully **credited**.
+
+Please read and understand the contribution guide before creating an issue or pull request.
+
+## Viability
+
+When requesting or submitting new features, first consider whether it might be useful to others. Open
+source projects are used by many developers, who may have entirely different needs to your own. Think about
+whether or not your feature is likely to be used by other users of the project.
+
+## Procedure
+
+Before filing an issue:
+
+- Attempt to replicate the problem, to ensure that it wasn't a coincidental incident.
+- Check to make sure your feature suggestion isn't already present within the project.
+- Check the pull requests tab to ensure that the bug doesn't have a fix in progress.
+- Check the pull requests tab to ensure that the feature isn't already in progress.
+
+Before submitting a pull request:
+
+- Check the codebase to ensure that your feature doesn't already exist.
+- Check the pull requests to ensure that another person hasn't already submitted the feature or fix.
+
+## Requirements
+
+If the project maintainer has any additional requirements, you will find them listed here.
+
+- **[PSR-12 Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-12-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.md` and any other relevant documentation are kept up-to-date.
+
+- **Consider our release cycle** - We try to follow [SemVer v2.0.0](http://semver.org/). Randomly breaking public APIs is not an option.
+
+- **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](http://www.git-scm.com/book/en/v2/Git-Tools-Rewriting-History#Changing-Multiple-Commit-Messages) before submitting.
+
+**Happy coding**!