add CONTRIBUTING.md

Author: typicode

CONTRIBUTING.md

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+## Agreement
+
+Thanks for your interest in contributing!
+
+By contributing to this project, you agree to the following:
+
+1. **Relicensing:** to support the project's sustainability and ensure it's longevity, your contributions can be relicensed to any license.
+
+2. **Ownership Rights:** You confirm you own the rights to your contributed code.
+
+3. **Disagreement:** If you disagree with these terms, please create an issue instead. I'll handle the bug or feature request.
+
+4. **Benefits for Contributors:** If your contribution is merged, you'll enjoy the same benefits as a sponsor for one year. This includes using the project in a company context, free of charge.
+
+## Fair Source License
+
+This project uses the Fair Source License, which is neither purely open-source nor closed-source. It allows visibility of the source code and free usage for a limited number of two users within an organization. Beyond this limit (three or more users), a small licensing fee via [GitHub Sponsors](https://github.com/sponsors/typicode) applies.
+
+This doesn't apply to individuals, students, teachers, small teams, ...
+
+Got questions or need support? Feel free to reach out to [email protected].