In addition to the documentation auto-generated from inline source code comments, you will find a more systematic overview of Theseus itself and how to contribute to its development, in the form of a small book.
The book is written in Markdown and uses mdBook to build the book into a human-friendly form, e.g., HTML.
To compile the book's source files, install mdbook and in the documentation folder, run:
mdbook build
When it successfully finishes, you can read the book starting at the top-level file book/index.html.