docs: 📝 add overlay usage instructions for pkgs.gen-commit

Author: rodeyseijkens

README.md

@@ -155,6 +155,34 @@ nix shell github:rodeyseijkens/gen-commit -c gen-commit [OPTIONS]
 }
 ```
 
+### Using Overlay (`pkgs.gen-commit`)
+
+If you prefer using `pkgs.gen-commit`, add this flake's overlay:
+
+```nix
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    gen-commit.url = "github:rodeyseijkens/gen-commit";
+  };
+
+  outputs = { self, nixpkgs, gen-commit, ... }:
+  {
+    nixosConfigurations.yourHost = nixpkgs.lib.nixosSystem {
+      system = "x86_64-linux";
+      modules = [
+        {
+          nixpkgs.overlays = [gen-commit.overlays.default];
+          environment.systemPackages = [
+            pkgs.gen-commit
+          ];
+        }
+      ];
+    };
+  };
+}
+```
+
 ## 🧠 How it works
 
 The tool analyzes your git changes (staged, unstaged, or untracked files) and uses the `llm` command to generate an appropriate commit message following the Conventional Commits specification with gitmoji. You can optionally provide additional context using the `-n, --note` option to guide the LLM's message generation.