Add proper documentation for Nixpkgs

INSTALL.md

@@ -6,6 +6,34 @@ If you are a tpm user, you can install the theme and keep up to date by adding t
 
 	set -g @plugin 'dracula/tmux'
 
+### Install with [Nix](https://nixos.org)
+
+If you're using [home-manager](https://github.com/nix-community/home-manager), an example config would look similar to this:
+Then run `home-manager switch`, the `Activating theme` section doesn't apply here.
+
+```nix
+programs.tmux = {
+	enable = true;
+	clock24 = true;
+	plugins = with pkgs.tmuxPlugins; [
+		sensible
+		yank
+		{
+			plugin = dracula;
+			extraConfig = ''
+				set -g @dracula-show-battery false
+				set -g @dracula-show-powerline true
+				set -g @dracula-refresh-rate 10
+			'';
+		}
+	];
+
+	extraConfig = ''
+		set -g mouse on
+	'';
+};
+```
+
 #### Activating theme
 
 1. Make sure  `run -b '~/.tmux/plugins/tpm/tpm'` is at the bottom of your .tmux.conf