docs(nix): add systemd user service example

nix/README.md

@@ -0,0 +1,41 @@
+# Nix
+
+The flake exports the T3 Code package for `x86_64-linux`.
+
+## NixOS user service
+
+Add T3 Code to your flake inputs:
+
+```nix
+inputs.t3code.url = "github:tarik02-org/t3code";
+```
+
+Then add the package and user service to your NixOS configuration:
+
+```nix
+{ inputs, pkgs, ... }:
+
+let
+  t3code = inputs.t3code.packages.${pkgs.stdenv.hostPlatform.system}.default;
+in
+{
+  environment.systemPackages = [ t3code ];
+
+  systemd.user.services.t3code = {
+    description = "T3 Code server";
+    wantedBy = [ "default.target" ];
+    wants = [ "network-online.target" ];
+    after = [ "network-online.target" ];
+
+    serviceConfig = {
+      Type = "simple";
+      ExecStart = "${t3code}/bin/t3 serve --host 0.0.0.0 --port 3773";
+      WorkingDirectory = "%h";
+      Environment = [ "T3CODE_NO_BROWSER=1" ];
+      Restart = "on-failure";
+      RestartSec = "5s";
+      OOMPolicy = "continue";
+    };
+  };
+}
+```