TYPO3 CMS core is a volunteer effort. We encourage you to join the project by submitting changes.
Have a look at our detailed contribution walkthrough.
If you like to contribute code, please use the main git repository, all changes are processed in our review system. The GitHub repository is a synchronized mirror of the primary git repository, pull requests via GitHub will be moved to our review system automatically.
If you like to contribute to our documentation, use the "Edit me on GitHub" button in the top right of each manual or reference or just make a pull request in any of the repositories that can be found in the TYPO3 documentation team GitHub organisation.