I created an extension documentation with a README.rst as main documentation ("single file solution" because I wanted to check it out and also for minimal documentation I find this much more convenient (at least for myself). (And that is not because of problems of dealing with .rst or sphinx!).
I am not 100% sure I tested the "Edit on GitHub" before, so I don't know if this used to work. But it does not work now:
The generated link looks for a README.rst in Documentation (but the README.rst is in the root dir)