docs: update Nix usage instructions

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I8717b49f21f777003787a663feb0467a6a6a6964

README.md

@@ -232,6 +232,40 @@ npm start
 
 </details>
 
+<details>
+<summary>Nix</summary>
+
+**Flake** (NixOS or flake-enabled systems):
+
+```nix
+{
+  inputs.troutbot.url = "github:notashelf/troutbot";
+
+  outputs = { self, nixpkgs, troutbot }: {
+    nixosConfigurations.myhost = nixpkgs.lib.nixosSystem {
+      modules = [
+        troutbot.nixosModules.troutbot
+        {
+          services.troutbot = {
+            enable = true;
+            environmentFile = "/path/to/.env"; # use Agenix if possible
+            configPath = "/path/to/config.ts" # use Agenix if possible
+          };
+        }
+      ];
+    };
+  };
+};
+```
+
+**Run directly**:
+
+```bash
+nix run github:notashelf/troutbot
+```
+
+</details>
+
 <details>
 <summary>Docker</summary>