docs: shells for declarative package managers (guix and nix) (#1378)

README.md

@@ -84,8 +84,6 @@ Wry also needs [WebKitGTK](https://webkitgtk.org/) for WebView. So please make s
 sudo pacman -S webkit2gtk-4.1
 ```
 
-The `libayatana-indicator` package can be installed from the Arch User Repository (AUR).
-
 #### Debian / Ubuntu:
 
 ```bash
@@ -98,7 +96,42 @@ sudo apt install libwebkit2gtk-4.1-dev
 sudo dnf install gtk3-devel webkit2gtk4.1-devel
 ```
 
-Fedora does not have the Ayatana package yet, so you need to use the GTK one, see the [feature flags documentation](https://docs.rs/wry/latest/wry/#feature-flags).
+### Nix & NixOS
+
+ ```Nix
+# shell.nix
+
+ let
+    # Unstable Channel | Rolling Release
+    pkgs = import (fetchTarball("channel:nixpkgs-unstable")) { };
+    packages = with pkgs; [
+      pkg-config
+      webkitgtk_4_1
+    ];
+  in
+  pkgs.mkShell {
+    buildInputs = packages;
+  }
+ ```
+
+ ```sh
+ nix-shell shell.nix
+ ```
+
+#### GUIX
+
+ ```scheme
+;; manifest.scm
+
+ (specifications->manifest
+   '("pkg-config"                ; Helper tool used when compiling
+     "webkitgtk"                 ; Web content engine fot GTK+
+  ))
+ ```
+
+```sh
+guix shell -m manifest.scm
+````
 
 ### macOS