Add contributing.md

Author: rolandgeider

.github/contributing.md

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+# Contributing to wger
+
+🎉 Thanks for showing interest in contributing! 🎉
+
+We have centralized the documentation for contributing to wger in the online
+docs, especially for non-code contributions such as documentation, translations,
+etc.:
+
+<https://wger.readthedocs.io/en/latest/contributing.html>
+
+## Questions
+
+Are you just using the software and have a question or improvement? Let us know!
+
+* Discord: <https://discord.gg/rPWFv6W>
+* Mastodon: <https://fosstodon.org/@wger>
+
+## Issues
+
+If you run into a bug describe the problem as well as you can.
+
+- Steps and any useful information to reproduce the issue
+- If you use our server, the time when this happened, we might be able to get some
+  information from the logs
+- Environment details (app type: web / mobile, installation method, OS, etc.)
+- Screenshots or logs if applicable
+- Git SHA1 if installed from source