Display README.lhs if there is no README.md

[sellout]

markdown-unlit treats Markdown files as Literate Haskell, and it recommends using a README.lhs file symlinked from README.md. The README.lhs will already be included in the package due to it being used in a test suite, while the README.md would need to additionally be added via an extra-files entry in the Cabal file.

It would be nice if Hackage rendered the README.lhs as Markdown if no README.md file is included in the package.

The one issue with this is if the README.lhs is in LaTeX or some other format. However, GitHub doesn’t support LaTeX as a README format (and I think the same is true of other software forges), so I think this pattern is unlikely.

This is different from other README requests like #574 and #890 in that it doesn’t require any new rendering, just falling back to a different file name.

Unfortunately, Hackage doesn’t track test-suite dependencies, so it’s hard to tell how widespread this pattern is, but I know at least one package that has a README.lhs without a README.md in the Cabal package that would benefit from this (and I would like to do the same with my own packages).