Adding the install command on Nix/NixOS

Installation.md

@@ -49,6 +49,20 @@ Now proceed to [Manually Install Radarr](/Radarr/Radarr/wiki/Installation#manual
 
     aurman -S radarr
 
+### Nix/NixOS ###
+
+Using Nix, the installation of [the package](https://nixos.org/nixos/packages.html?attr=radarr&channel=nixpkgs-unstable&query=radarr) (via your stable channel) is possible via the following command : 
+
+	nix-env -iA nixpkgs.radarr
+
+On NixOS you have two choices : 
+* install only the package (via the nixos channel) :
+	`nix-env -iA nixos.radarr`
+* activate the option (recommended option) by adding the following line to your `configuration.nix` :
+	`services.radarr.enable = true;`
+
+If you choose the option route, do check out [the parameters](https://nixos.org/nixos/options.html#radarr) that you can adjust to your liking.
+
 ### Manually Install Radarr ###
 
 Download and extract the latest [release](https://github.com/Radarr/Radarr/releases) from github, changing the url below to match.