README.rst: add nix docs

README.rst

@@ -117,7 +117,6 @@ Scoop
 
     scoop install gallery-dl
 
-
 Homebrew
 --------
 
@@ -174,6 +173,48 @@ This will remove the container after every use so you will always have a fresh e
 
 You can also add an alias to your shell for "gallery-dl" or create a simple bash script and drop it somewhere in your $PATH to act as a shim for this command.
 
+Nix and Home Manager
+--------------------------
+
+Adding *gallery-dl* to your system environment:
+
+.. code:: nix
+
+    environment.systemPackages = with pkgs; [
+      gallery-dl
+    ];
+
+Using :code:`nix-shell`
+
+.. code:: bash
+
+    nix-shell -p gallery-dl
+
+.. code:: bash
+
+    nix-shell -p gallery-dl --run "gallery-dl <args>"
+
+For Home Manager users, you can manage *gallery-dl* declaratively:
+
+.. code:: nix
+
+    programs.gallery-dl = {
+      enable = true;
+      settings = {
+        extractor.base-directory = "~/Downloads";
+      };
+    };
+
+Alternatively, you can just add it to :code:`home.packages` if you don't want to manage it declaratively:
+
+.. code:: nix
+
+    home.packages = with pkgs; [
+      gallery-dl
+    ];
+
+After making these changes, simply rebuild your configuration and open a new shell to have *gallery-dl* available.
+
 Usage
 =====