Improve readme

There is only setup instructions and maintainer instructions for dealing with secrets. There's nothing about adding or importing repositories. Add missing information, including an example that can be copied to add a new repository.

Start by reading the README's existing setup and maintainer instructions for handling secrets. Add the missing guidance for adding or importing repositories, including a copyable example for adding a new repository. Done means the README explains both workflows clearly without requiring undocumented steps.

Written by the indexing model from the issue text.