Add in doco for hie-nix pinning per project

[benkolera]

I installed hie-nix globally and got surprised when my hie in spacemacs broke on a haskell project pinned to an older nix pkgs than my nixos environment. Code compiled with ghc-8.4.3 cannot be read by a ghc-8.4.4 hie.

If you are pinning nixpkgs for a project, you want to pin hie-nix for a project. If all of your haskell packages just use the system nixpkgs installing globally should usually be fine (except when hie-nix is just behind nixpkgs. Unless hie-nix started producing hie-8.4.4. binaries as well as the hie-8.4 that it does now. hie-wrapper would do the right thing if hie-nix did that.