Add contributing guidelines and pull request template

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
+- Environment details (app type: web / mobile, installation method, OS, etc.)
+- Any changes you may have done, either to the compose file or the configuration
+- Any logs if applicable
+- Git SHAs of this checkout, docker images, etc.

.github/pull_request_template.md

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+# Proposed Changes
+
+-
+-
+
+## Related Issue(s)
+
+If applicable, please link to any related issues (`Closes #123`,
+`Closes wger-project/other-repo#123`, `See also #123`, etc.)