feat: Add example config

Will extend upon this later, but this basically makes it possible to
guarantee that the flake can be used inside another flake and be
customized as expected.

Note that hardware-configuration.nix is optional if you're just using
the configuration for virtual machines and containers, which is why it's
optional here. A default file isn't provided to help users understand
that they have to bring their own.

example/configuration.nix

@@ -0,0 +1,43 @@
+{ nix-config, pkgs, ... }:
+
+let
+  inherit (builtins) attrValues;
+in
+{
+  imports = attrValues {
+    inherit (nix-config.nixosModules) system shell;
+
+    customConfig = {
+      modules.system.username = "demo";
+    };
+  };
+
+  home-manager.sharedModules = attrValues {
+    inherit (nix-config.homeManagerModules) yazi;
+
+    youCanNameThisAnything = {
+      programs.btop.enable = true;
+    };
+  };
+
+  environment.systemPackages = attrValues {
+    inherit (nix-config.packages.x86_64-linux) webp-thumbnailer;
+
+    inherit (pkgs) ruby php;
+  };
+
+  nixpkgs.overlays = attrValues {
+    inherit (nix-config.overlays) kitty;
+
+    exampleOverlay = final: prev: {
+      btop = prev.btop.overrideAttrs (oldAttrs: {
+        postInstall = (oldAttrs.postInstall or "") + /* bash */ ''
+          echo "#!/usr/bin/env sh"  >> btop-overlay
+          echo "echo 'hello world'" >> btop-overlay
+
+          install -Dm755 btop-overlay $out/bin/btop-overlay
+        '';
+      });
+    };
+  };
+}

example/flake.nix

@@ -0,0 +1,25 @@
+{
+  description = "An example of creating your own flake that extends this nix-config";
+
+  inputs = {
+    nix-config.url = "github:donovanglover/nix-config";
+  };
+
+  outputs = { nix-config, ... } @ attrs:
+    let
+      inherit (nix-config.inputs) nixpkgs;
+      inherit (nixpkgs.lib) nixosSystem optional;
+      inherit (builtins) pathExists;
+    in
+    {
+      nixosConfigurations = {
+        hyprland = nixosSystem {
+          system = "x86_64-linux";
+          specialArgs = attrs;
+          modules = [
+            ./configuration.nix
+          ] ++ optional (pathExists ./hardware-configuration.nix) ./hardware-configuration.nix;
+        };
+      };
+    };
+}