doc: Add example flake

example/flake.nix

@@ -0,0 +1,39 @@
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    tf2-nix.url = "path:///home/crumb/src/tf2-nix";
+  };
+
+  outputs = { self, ... }@inputs: {
+    # TF2 config built as a package.
+    packages.x86_64-linux.default =
+      let
+        pkgs = import inputs.nixpkgs { system = "x86_64-linux"; };
+        tf2pkgs = inputs.tf2-nix.packages.x86_64-linux;
+      in tf2pkgs.mergeTf2Configs (with tf2pkgs; [
+        mastercomfig.presets.medium-low
+        mastercomfig.addons.flat-mouse
+        mastercomfig.addons.no-tutorial
+        mastercomfig.addons.null-canceling-movement
+        maps.jump_beef
+        improved-crosshairs
+        loadouts-script
+        # Packages are overridable.
+        (huds.deerhud.overrideAttrs (final: prev: {
+          patches = [ ./raise-uber-meter.patch ];
+        }))
+        # Existing configs on your file system may be referenced by path:
+        ./my-config
+      ]);
+
+    # TF2 config installed using Home-manager.
+    homeConfigurations.default =
+      let gameDir = ".local/share/Steam/steamapps/common/Team Fortress 2/tf";
+      in {
+        home.file.${gameDir} = {
+          recursive = true;
+          source = self.packages.x86_64-linux.default;
+        };
+      };
+  };
+}