Use a custom README file for the GitHub package rather than the main project README

[eguiraud]

Description

Hi,
we have a monorepo that produces several images and pushes them to the github registry.

They all pick up the main repository's readme, which gets displayed as the package's readme at https://github.com/project/packages.
It would be great if we could specify one readme per image, or if the action automatically picked up a README.md placed next to the dockerfile.

This is a continuation of as a continuation of #722 in which this part of the issue didn't get resolved.