Add CONTRIBUTING

CONTRIBUTING.md

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+
+Contributing to DAVx⁵
+=====================
+
+**Thank you for your interest in contributing to DAVx⁵!**
+
+Because you're reading this, you're probably interested in
+contributing to the DAVx⁵ code. [Other ways to contribute:
+see here.](https://www.davx5.com/donate#c306)
+
+To contribute:
+
+1. It's good idea to have a look at the [DAVx⁵ Roadmap](https://gitlab.com/bitfireAT/davx5-ose/wikis/Roadmap)
+   to see whether the change is already planned. Maybe there's even a link to a
+   corresponding forum thread there.
+1. Determine which project the changes shall go to. There's
+   the DAVx⁵ main project (this repo), and the [related
+   libraries](README.md).
+1. Please post to the [DAVx⁵ development forum](https://www.davx5.com/forums)
+   before doing actual work (unless you do it only for yourself, of course).
+   This will help to coordinate activities and you'll also get hints
+   about where to start and possible pitfalls.
+1. Fork the repository.
+1. Do the changes in your repository.
+1. Submit a pull request to the original project.
+1. Post in the forum again (to make sure the pull request is being notified).
+
+
+Questions, discussion
+=====================
+
+We're happy to see questions, discussions etc. in the
+[DAVx⁵ development forum](https://www.davx5.com/forums)!
+
+
+Licensing
+=========
+
+All code has to be licensed under the GPL.
+
+We (bitfire.at, initial developers) are also asking you to double-license the
+code so that we can also use it for related non-open source projects like
+[Managed DAVx⁵](https://www.davx5.com/organizations/managed-davx5).
+
+Please find more about this in the Contributor's License Agreement (CLA)
+we'll send to you if you want to contribute.
+