Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+Suggestions or contributions to improve the contents are very welcome. 
+
+## How to contribute
+
+### Issues
+The easiest way to contribute is to submit an issue and make your comments about contents, typos or any other suggestions.
+
+### Pull requests
+If you are comfortable with git and pull requests, you can also submit a pull request where you directly suggest changes to the content. Read more about how pull requests work [here](https://app.egghead.io/playlists/how-to-contribute-to-an-open-source-project-on-github).
+
+In short:
+
+1. Fork the repository and clone it locally.
+2. Create a new branch in your desktop copy of this repository.
+3. Commit the change in that branch.
+4. Push that branch to your fork of this repository on GitHub
+5. Submit a pull request from that branch to the main branch of the master repository. 
+6. If you receive feedback, make changes on your desktop and push to your branch on GitHub: the pull request will update automatically.
+
+### Email
+You can reach the author/maintainer at e.b.jarochowska [at] uu.nl