Add support for overriding the README-recognizing regex

[TomasHubelbauer]

This can be used to configure the plugin to use one of the alternative READMEs GitHub supports or even any other custom file!

https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-readmes

If you put your README file in your repository's hidden .github, root, or docs directory, GitHub will recognize and automatically surface your README to repository visitors.

If a repository contains more than one README file, then the file shown is chosen from locations in the following order: the .github directory, then the repository's root directory, and finally the docs directory.

Closes #38