Include README.md in Cabal package

[axman6]

No description provided.

[aspiwack]

Thanks.

I suppose it's a reasonable thing to do (I'm saying this because I probably never add the README file that way, which probably mean I'm a bad person (Pup, for instance doesn't do that either)). But we can't do this change in the Cabal file only, since this repo uses Hpack: we need to change the package.yaml file as well.

[axman6]

I'll have to see how stack does things, it's something I avoid usually.

[aspiwack]

This should fix it. Thanks for educating me.