Add readme for nix usage

README.md

@@ -5,3 +5,28 @@ Windows XP as your desktop background.
 This embeds a GTK-based webview to use https://winxp.vercel.app/ as your desktop background.
 
 Only works on Wayland with gtk-layer-shell.
+
+## Nix
+
+1. Import the repo as an overlay:
+
+```nix
+let
+  nixpkgs = import nixpkgs {
+    inherit system;
+    overlays = [
+      (import winxpbg)
+    ];
+  };
+in
+...
+```
+2. Launch it with your desktop environment or window manager, for example with Niri:
+
+```nix
+{
+  programs.niri.settings.spawn-at-startup = [
+    { command = [ (lib.getExe pkgs.winxpbg) ]; }
+  ];
+}
+```