As it stands, the only documentation for how to generate reference packages is in the usage output of the generate script:
https://github.com/dotnet/source-build-reference-packages/blob/8c072e50c9ff73a1d3ca791df7d9722004af49ce/generate.sh#L23-L42

This doesn't explain things like the build order, dependencies, or how to solve build issues. The documentation should describe exactly how to add packages, how to handle dependencies and build ordering, and address common pitfalls.