document nixpkgs package

Author: outfoxxed

src/guide/install-setup.mdx

@@ -18,8 +18,9 @@ Note that you may want to install some additional packages (names vary by distro
 - `qt5compat`: extra visual effects, notably gaussian blur. @@QtQuick.Effects.MultiEffect is usually preferable
 
 ### Nix
-The Quickshell repo has an embedded flake. You can use either of the two:
+Release versions of Quickshell are available from Nixpkgs under the `quickshell` package.
 
+The Quickshell repo also has an embedded flake which can be used from either mirror:
 - `git+https://git.outfoxxed.me/outfoxxed/quickshell`
 - `github:quickshell-mirror/quickshell`