downloads: Add entry for NixOS owned by maralorn

site/downloads.markdown

@@ -71,6 +71,18 @@ Alternatively, many operating systems provide GHC, cabal and Stack through their
 
 **Do not use the Haskell development tools provided by Arch, they are broken.** For more information see [[1]](https://dixonary.co.uk/blog/haskell/cabal-2020) [[2]](https://stackoverflow.com/questions/65643699/what-is-the-suggested-way-of-setting-up-haskell-on-archlinux/65644318#65644318).
 
+#### NixOS
+
+[](This installation method is owned by @maralorn)
+
+* [Documentation for Haskell development and packaging with nix](https://haskell4nix.readthedocs.io/nixpkgs-users-guide.html#how-to-create-a-development-environment)
+* **Using ghcup on NixOS does not work.** Use the official packages instead.
+* Official packages
+    [GHC](https://search.nixos.org/packages?show=ghc&type=packages&query=ghc),
+    [cabal](https://search.nixos.org/packages?show=cabal-install&type=packages&query=cabal-install),
+    [HLS](https://search.nixos.org/packages?show=haskell-language-server&type=packages&query=haskell-language-server),
+    [Stack](https://search.nixos.org/packages?show=stack&type=packages&query=stack)
+
 </div>
 
 <p><a data-toggle="collapse" href="#collapse-freebsd" class="btn btn-xs btn-primary">Show FreeBSD packages</a></p>