add contributing file

Author: mikaelcom

CONTRIBUTING.md

@@ -0,0 +1,28 @@
+# Contributing
+
+Contributions are **welcome** and will be fully **credited**.
+
+We accept contributions via pull requests on [Github].
+Please make all pull requests to the `develop` branch, not the `master` branch.
+
+## Pull Requests
+
+- **Create an issue** - Explain as detailed as possible the issue you encountered so we can understand the context of your pull request
+- **[Symfony Coding Standard]** - The easiest way to apply the conventions is to run `composer lint`
+- **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.
+- **Create feature 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
+$ composer test
+```
+
+**Happy coding**!
+
+[Github]: https://github.com/wsdltophp/packagebase
+[Symfony Coding Standard]: http://symfony.com/doc/current/contributing/code/standards.html