Occasionally tool authors use README.md or similar.

Planemo lint could warn if the preferred form README.rst is not present (is that the IUC preference? We should say so on https://wiki.galaxyproject.org/Tools/BestPractices if so)

Planemo lint could error in there is no recognised readme file at all (README.txt, repository_name.txt, or README.rst, etc - using same listing as the Tool Shed code).