docs(readme): add nixpkgs details

Resolves #303.

README.md

@@ -75,7 +75,15 @@ cargo install ironbar
 yay -S ironbar-git
 ```
 
-### Nix Flake
+### Nix
+
+[nix package](https://search.nixos.org/packages?channel=unstable&show=ironbar)
+
+```sh
+nix-shell -p ironbar
+```
+
+#### Flake
 
 A flake is included with the repo which can be used with Home Manager.
 
@@ -170,4 +178,4 @@ All are welcome, but I ask a few basic things to help make things easier. Please
 - [Waybar](https://github.com/Alexays/Waybar) - A lot of the initial inspiration, and a pretty great bar.
 - [Rustbar](https://github.com/zeroeightysix/rustbar) - Served as a good demo for writing a basic GTK bar in Rust
 - [Smithay Client Toolkit](https://github.com/Smithay/client-toolkit) - Essential in being able to communicate to Wayland
-- [gtk-layer-shell](https://github.com/wmww/gtk-layer-shell) - Ironbar and many other projects would be impossible without this
+- [gtk-layer-shell](https://github.com/wmww/gtk-layer-shell) - Ironbar and many other projects would be impossible without this