Building the dependency docs lets Haddock generate links into them.

This lets you convince cabal to use the same version of dependencies that you
are managing with nix.

This is nice for in-development versions of dependencies or things that just
aren't available on hackage yet.

Add "release" and "cabal-test" app helpers

See merge request jcalderone/hs-flake-utils!3

add a `shellHook` parameter to `devShells`

See merge request jcalderone/hs-flake-utils!2

It gets added to the pre-commit stuff.

This will let downstream projects follow our nixpkgs to increase cache sharing.

add a getting started section to the readme

See merge request jcalderone/hs-flake-utils!1

This is based heavily on
https://github.com/JonathanLorimer/templates/blob/main/template/haskell/flake.nix
but I like libraries a lot more than I like code generation.

There's some more pieces that are probably useful commented out.