I just noticed that Greg Landrum has posted a page on how to contribute to RDKit. https://github.com/rdkit/rdkit/wiki/HowToContribute.

There many ways to contribute, you don't have to be Python or C++ developer, simply being an active user and asking questions and contributing solutions helps other users. Improving the documentation is always a great place from newcomers to start, particularly highlighting things that are not as clear as they could be.

I've also added the link to the Toolkits page.