Add NIX.md

There are multiple ways one could use XMonad with nix, so a separate
file for documenting all of this seems appropriate.  E.g., when it
contains the respective installation instructions (e.g, "normally" as
instructed in the NixOS wiki, via stack, via flake, ...) we could link
to it from INSTALL.md.

For now it's a good place to document the new `develop.nix`
functionality.

Co-authored-by: Ivan Malison <IvanMalison@gmail.com>

NIX.md

@@ -0,0 +1,17 @@
+# `nix` integration for XMonad
+
+## Customizing the `nix-shell`
+
+It's possible to use a file `develop.nix` to customize the `devShell`
+provided by the flake.  This is useful if e.g. you want to have the
+`haskell-language-server` or other developer tools in the shell properly
+configured (correct GHC versions, and the like).
+
+Here is an example `develop.nix` for `haskell-language-server`:
+
+``` nix
+pkgs: devInputs: devInputs // {
+  nativeBuildInputs = with pkgs.haskellPackages;
+    [ cabal-install hlint ghcid ormolu implicit-hie haskell-language-server ];
+}
+```

xmonad-contrib.cabal

@@ -23,7 +23,7 @@ license:            BSD3
 license-file:       LICENSE
 author:             Spencer Janssen & others
 maintainer:         xmonad@haskell.org
-extra-source-files: README.md CHANGES.md scripts/generate-configs scripts/run-xmonad.sh
+extra-source-files: README.md CHANGES.md NIX.md scripts/generate-configs scripts/run-xmonad.sh
                     scripts/window-properties.sh
                     scripts/xinitrc scripts/xmonad-acpi.c
                     scripts/xmonad-clock.c