docs: add NixOS installation instructions to README

Add NixOS package installation section under a new "Package Managers"
category in the installation instructions. This reflects tenere's
recent addition to nixpkgs.

Resolves pythops#43
See NixOS/nixpkgs#363556

README.md

@@ -43,6 +43,21 @@ You can download the pre-built binaries from the [release page](https://github.c
 ```shell
 cargo install tenere
 ```
+### Package Managers
+
+#### NixOS / Nix
+=======
+### 🗃️ Package Managers
+
+#### :snowflake: NixOS / Nix
+
+Tenere is available in nixpkgs and can be installed by adding it to your system packages:
+
+```nix
+environment.systemPackages = [
+  pkgs.tenere
+];
+```
 
 ### ⚒️ Build from source