Added contribution guide

Author: bhermann

CONTRIBUTING.md

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+# Contribution Guide
+
+We follow the GitHub [Fork & Pull][forkandpull] and [Git Flow][gitflow] workflow in this project.
+Please fork the official repository, develop in a branch based on the
+current develop branch, and submit a pull request (PR) after you are done.
+
+[forkandpull]: https://help.github.com/articles/about-pull-requests/
+[gitflow]: http://nvie.com/posts/a-successful-git-branching-model/
+
+## Checklist before submitting a Pull Request
+
+Before you submit your PR, please go through this list and check if
+your request fulfills these points.
+
+- Do you have more changes/additions/deletions in your PR than you expected?
+- Do you have tests for your changes?
+- Do all tests of the project succeed?
+- Is your pull request based on the `develop` branch? [(cf. GitFlow)][gitflow]
+
+If you check these items before, your pull request is more likely to be
+included into the project quickly.
+