nix: add NixOS module and tests (#2857)

nix/README.md

@@ -0,0 +1,41 @@
+# Headscale NixOS Module
+
+This directory contains the NixOS module for Headscale.
+
+## Rationale
+
+The module is maintained in this repository to keep the code and module
+synchronized at the same commit. This allows faster iteration and ensures the
+module stays compatible with the latest Headscale changes. All changes should
+aim to be upstreamed to nixpkgs.
+
+## Files
+
+- **[`module.nix`](./module.nix)** - The NixOS module implementation
+- **[`example-configuration.nix`](./example-configuration.nix)** - Example
+  configuration demonstrating all major features
+- **[`tests/`](./tests/)** - NixOS integration tests
+
+## Usage
+
+Add to your flake inputs:
+
+```nix
+inputs.headscale.url = "github:juanfont/headscale";
+```
+
+Then import the module:
+
+```nix
+imports = [ inputs.headscale.nixosModules.default ];
+```
+
+See [`example-configuration.nix`](./example-configuration.nix) for configuration
+options.
+
+## Upstream
+
+- [nixpkgs module](https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/networking/headscale.nix)
+- [nixpkgs package](https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/he/headscale/package.nix)
+
+The module in this repository may be newer than the nixpkgs version.