docs: add NixOS module documentation to README

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -148,6 +148,60 @@ When running as an MCP server, the following tools are available:
 | `list_revisions` | List all indexed revisions |
 | `delete_revision` | Delete an indexed revision |
 
+## NixOS Module
+
+A NixOS module is provided for running the MCP server as a systemd service.
+
+```nix
+{
+  inputs.labmcp.url = "github:torjus/labmcp";
+
+  outputs = { self, nixpkgs, labmcp }: {
+    nixosConfigurations.myhost = nixpkgs.lib.nixosSystem {
+      system = "x86_64-linux";
+      modules = [
+        labmcp.nixosModules.nixos-options-mcp
+        {
+          services.nixos-options-mcp = {
+            enable = true;
+            indexOnStart = [ "nixos-unstable" ];
+          };
+        }
+      ];
+    };
+  };
+}
+```
+
+### Module Options
+
+| Option | Type | Default | Description |
+|--------|------|---------|-------------|
+| `enable` | bool | `false` | Enable the service |
+| `package` | package | from flake | Package to use |
+| `database.type` | enum | `"sqlite"` | `"sqlite"` or `"postgres"` |
+| `database.name` | string | `"nixos-options.db"` | SQLite database filename |
+| `database.connectionString` | string | `""` | PostgreSQL connection URL |
+| `indexOnStart` | list of string | `[]` | Revisions to index on service start |
+| `user` | string | `"nixos-options-mcp"` | User to run the service as |
+| `group` | string | `"nixos-options-mcp"` | Group to run the service as |
+| `dataDir` | path | `/var/lib/nixos-options-mcp` | Directory for data storage |
+
+### PostgreSQL Example
+
+```nix
+{
+  services.nixos-options-mcp = {
+    enable = true;
+    database = {
+      type = "postgres";
+      connectionString = "postgres://nixos:nixos@localhost/nixos_options?sslmode=disable";
+    };
+    indexOnStart = [ "nixos-unstable" "nixos-24.11" ];
+  };
+}
+```
+
 ## Development
 
 ```bash