A description of the file system structure of the repository. Explains how things are organized, where existing stuff is located and where to add new stuff.
I run tree on the repository root, append the output to the README and add comments to every line explaining every directory and sometimes every file.
* lots of screenshots
* a README.md that is extremely long and detailed
* tutorials, reference, design documents, architecture diagrams
* mental model documents to explain how things are thought of by the authors