You can contribute to the Godot .NET integration with issues and PRs. Just filling issues for problems you encounter or triaging existing issues is a great way to contribute. Contributing implementations is greatly appreciated.
To know more about how to contribute, please read the contributors guidelines in the main repository for the Godot engine first.
Then, familiarize yourself with this repository. There are README.md files all over the repository that describe how everything works. To begin you'll want to learn how to build the repository, take a look at the README.md at the root of this repository for instructions.
Feel free to join us in the #dotnet channel in the Godot Contributors Chat and ask any questions you may have.
Thanks for your interest in contributing!