Contributions are obviously welcome. The easiest way to contribute is to simply make a pull request.
There is some basic developer documentation at the website.
Contributed code and assets will be assumed to follow the overall license split as the original code. That is: code will be assumed to be contributed under GPL and assets will be assumed to be contributed under CC0.
You are obviously only allowed to contribute code that you own or are permitted to share. As this project is dedicated to remain a community effort, no explicit copyright transfer is requested.