[TASK] Add contribution guide (#1044)

CONTRIBUTING.md

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+# How to contribute
+
+## Code of Conduct
+
+This project uses the
+[TYPO3 Code of Conduct](https://typo3.org/community/values/code-of-conduct).
+
+When you contribute to this project or interact with community members,
+you agree to adhere to this code of conduct.
+
+## Open an issue
+
+Feel free to open an issue - no matter whether you found a bug, would like to request a feature or have questions.
+
+1. Please check existing open and closed issues first and feel free to comment and re-open them.
+   Existing issues are available at https://github.com/FriendsOfTYPO3/tea/issues .
+2. Create a new issue at https://github.com/FriendsOfTYPO3/tea/issues/new .
+3. Use a meaningful title.
+4. Provide enough context, a concrete use case and steps to reproduce bugs.
+
+## Contributing code and documentation
+
+1. Create a fork of the repository.
+   You need a [GitHub account](https://github.com/join).
+2. Create a new branch holding your changes.
+3. Apply your changes.
+4. Commit your changes following the [TYPO3 git commit conventions](https://docs.typo3.org/m/typo3/guide-contributionworkflow/main/en-us/Appendix/CommitMessage.html#commitmessage).
+   The only relevant difference is that we do not use the `Releases` or `Change-Id` metadata.
+5. Push your changes.
+6. Open a pull request.
+
+Some pull requests can not be merged quickly.
+In cases where the changes cannot be merged quickly, we consider the original author responsible for keeping their PR branches up to by rebasing.
+
+Please also have a look at the documented [contribution workflow for GitHub](https://docs.github.com/en/get-started/quickstart/contributing-to-projects).