Create CONTRIBUTING.md

Author: eyarz

CONTRIBUTING.md

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+# How to contribute
+
+We like to encourage you to contribute to the repository.
+This should be as easy as possible for you but there are a few things to consider when contributing.
+The following guidelines for contribution should be followed if you want to submit a pull request.
+
+## How to prepare
+
+* Submit an [issue ticket](https://github.com/datreeio/node-datreeio/issues) for your issue if there is no one yet.
+* Fork the repository on GitHub
+
+## Make Changes
+
+* In your forked repository, create a topic branch for your upcoming patch.
+* Create a branch based on master.
+* Make sure you stick to the coding style that is used already.
+* Make commits of logical units and describe them properly.
+* Check for unnecessary whitespace with `git diff --check` before committing
+* If possible, submit tests to your patch / new feature so it can be tested easily.
+* Assure nothing is broken by running all the tests.
+
+## Submit Changes
+
+* Push your changes to a topic branch in your fork of the repository.
+* Open a pull request to the original repository and choose the right original branch you want to patch.
+* If not done in commit messages (which you really should do) please reference and update your issue with the code changes. 
+* Even if you have write access to the repository, do not directly push or merge pull-requests. Let another team member review your pull request and approve.
+
+# Additional Resources
+
+* [General GitHub documentation](http://help.github.com/)
+* [GitHub pull request documentation](https://help.github.com/articles/about-pull-requests/)