Add contributing guidelines

CONTRIBUTING.md

@@ -0,0 +1,11 @@
+CONTRIBUTING
+============
+
+Contributions are welcome, and are accepted via pull requests. Please review these guidelines before submitting any pull requests.
+
+## Guidelines
+
+* Please follow the [PSR-2 Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md) and [PHP-FIG Naming Conventions](https://github.com/php-fig/fig-standards/blob/master/bylaws/002-psr-naming-conventions.md).
+* Remember that we follow [SemVer](http://semver.org). If you are changing the behaviour, or the public api, you may need to update the docs.
+* Send a coherent commit history, making sure each individual commit in your pull request is meaningful. If you had to make multiple intermediate commits while developing, please [squash](http://git-scm.com/book/en/Git-Tools-Rewriting-History) them before submitting.
+* You may also need to [rebase](http://git-scm.com/book/en/Git-Branching-Rebasing) to avoid merge conflicts.