Adds hpack generated cabal file.

[philderbeast]

Should I check in generated cabal files?
Yes, you should. This recommendation was changed in issue #5210, please see the discussion there.

[tomjaguarpaw]

My personal point of view is that we should be wary of checking in generated files (the Stack advise notwithstanding) and if we do their consistency must be checked by CI. As a non-Stack user I sympathise with those who download a repo with no .cabal file in it and then wonder how to build it. Perhaps we can explain the hpack workflow somewhere.