doc: add a more complete flakes fragment for quickstart

docs/QUICK_START.md

@@ -125,16 +125,32 @@ Below is a fragment of a NixOS configuration that enables the Secure
 Boot stack.
 
 ```nix
-nixosConfigurations = {
+{
+  description = "A SecureBoot-enabled NixOS configurations";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+    lanzaboote = {
+      url = "github:nix-community/lanzaboote";
+
+      # Optional but recommended to limit the size of your system closure.
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = { self, nixpkgs, lanzaboote, ...}: {
+    nixosConfigurations = {
       yourHost = nixpkgs.lib.nixosSystem {
         system = "x86_64-linux";
 
         modules = [
-      # ... other modules ...
+          # This is not a complete NixOS configuration and you need to reference
+          # your normal configuration here.
 
           lanzaboote.nixosModules.lanzaboote
 
-      ({ config, pkgs, lib, ... }: {
+          ({ pkgs, lib, ... }: {
             # This should already be here from switching to bootspec earlier.
             # It's not required anymore, but also doesn't do any harm.
             boot.bootspec.enable = true;
@@ -157,6 +173,9 @@ nixosConfigurations = {
           })
         ];
       };
+    };
+  };
+}
 ```
 
 After you rebuild your system, check `sbctl verify` output: