Add Contributing guide

Author: moltam

CONTRIBUTING.md

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+# Contributing
+
+Contributions are welcome.
+
+We accept contributions via Pull Requests on [Github](https://github.com/lajax/yii2-language-picker).
+
+
+## Pull Requests
+
+- **[PSR-2 Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md)** - Check the code style with ``$ composer cs-check`` and fix it with ``$ composer cs-fix``.
+
+- **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](https://chris.beams.io/posts/git-commit/). 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**!