feat(docs): Add CONTRIBUTING.md

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+[title-hard-tab]
+[title-match-regex]
+regex=^((feat|fix|chore|refactor|docs|release|test)\((meta|tests|style|docs|xep|core)+(,(meta|tests|style|docs|xep|core))*\)|release): [A-Z0-9].*$
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CONTRIBUTING.md

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+# Contribution Guide
+
+Thanks for your interest in the moxxmpp XMPP library! This document contains guidelines and guides for working on the moxxmpp codebase.
+
+## Contributing
+
+If you want to fix a small issue, you can just fork, create a new branch, and start working right away. However, if you want to work
+on a bigger feature, please first create an issue (if an issue does not already exist) or join the [development chat](xmpp:moxxy@muc.moxxy.org?join) (xmpp:moxxy@muc.moxxy.org?join)
+to discuss the feature first.
+
+Before creating a pull request, please make sure you checked every item on the following checklist:
+
+- [ ] I formatted the code with the dart formatter (`dart format`) before running the linter
+- [ ] I ran the linter (`dart analyze`) and introduced no new linter warnings
+- [ ] I ran the tests (`dart test`) and introduced no new failing tests
+- [ ] I used [gitlint](https://github.com/jorisroovers/gitlint) to ensure propper formatting of my commig messages
+
+If you think that your code is ready for a pull request, but you are not sure if it is ready, prefix the PR's title with "WIP: ", so that discussion
+can happen there. If you think your PR is ready for review, remove the "WIP: " prefix.