Add CONTRIBUTING.md (#1201)

Added a CONTRIBUTION.md page for the project.

Contains a link to the wiki with more detailed description of the contribution workflow.

CONTRIBUTING.md

+## Contributing to Arbor
+
+First off, thank you for considering contributing to Arbor! It's people like you that
+make Arbor such a great tool. Feel welcome and read the following sections in order to
+know how to ask questions and how to work on something.
+
+There are many ways to contribute: writing tutorials or blog posts, improving the
+documentation, submitting bug reports and feature requests or writing code which can be
+incorporated into Arbor itself.
+
+[On our wiki](https://github.com/arbor-sim/arbor/wiki/How-to-contribute) we have laid out how you
+can go about making your contribution.
+
+## Get in touch
+
+You can reach out in the following ways:
+
+* [Discussions](https://github.com/arbor-sim/arbor/discussions). Any questions or remarks regarding using Arbor
+for your research are welcome.
+* [Slack](https://mcnest.slack.com). If you're interested in developing Arbor itself, you can visit our Slack.
+* [Email](mailto:arbor-sim@fz-juelich.de).