Motivation for this change

I noticed that the docs didn't mention anything regarding hackage2nix or how to go about editing hackage-packages.nix, so I added a new entry to "Misc" in the haskell infrastructure docs.

The markdown formatting is kind of ugly because I tried to keep it at 80 cols. Happy to fix that if needed :)

Cheers

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.