cabal check should warn when readme and other dev docs file exist in the working tree, but are not included in extra-doc-files

[ffaf1]

Offspring of #3964.

Describe the feature request
Browsing Hackage or flora.pm would be a much nicer experience if everyone added their devel docs to extra-doc-files.
Sometimes this is not the case, because people forget or because they choose not to (as rendering on Hackage might be off).

It would be nice if cabal check looked for all such files in the working tree and hinted the author to add them.
As now cabal check warns about changelogs but not other files (e.g. READMEs).

The check would have to be granular, so users can --ignore specific warnings.