added contribution guide

Author: ionut-tanasa

CONTRIBUTING.md

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+# Contributing
+
+Contributions are **welcome** and will be fully **credited**.
+
+
+## Pull Requests
+
+- **[PSR-2 Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md)**.
+
+- **Document any change in behaviour** - Make sure the `README.md` and any other relevant documentation are kept up-to-date.
+
+- **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.
+
+- **Pull requests go against develop branch** - So we can test it before merging it onto master.
+
+- **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.