docs: add NixOS installation section

Author: DerGrumpf

README.md

@@ -28,6 +28,19 @@ Or build directly from `master`:
 cargo install --git https://github.com/hecrj/icebreaker.git
 ```
 
+### NixOS
+
+Icebreaker is packaged in [nixpkgs](https://github.com/NixOS/nixpkgs) (maintained by [DerGrumpf](https://github.com/DerGrumpf)) as `icebreaker-chat` (the name `icebreaker` was already taken by an unrelated package):
+
+```bash
+nix run nixpkgs#icebreaker-chat
+```
+
+Or add `pkgs.icebreaker-chat` to your configuration's `environment.systemPackages`.
+
+> [!Note]
+> This package isn't in a stable NixOS release yet; use `nixpkgs-unstable` or `master` until it lands.
+
 ## Why "Icebreaker"?
 
 [Blame Mistral!](assets/naming.png)