added nix flake

flake.nix

@@ -0,0 +1,22 @@
+{
+  description = "rusty-pw: random word + number generator";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  outputs = { self, nixpkgs }:
+    let
+      systems = [ "x86_64-linux" "aarch64-linux" "aarch64-darwin" ];
+      forAllSystems = f:
+        nixpkgs.lib.genAttrs systems (system: f nixpkgs.legacyPackages.${system});
+    in
+    {
+      packages = forAllSystems (pkgs: rec {
+        rusty-pw = pkgs.callPackage ./package.nix { };
+        default = rusty-pw;
+      });
+
+      overlays.default = final: prev: {
+        rusty-pw = final.callPackage ./package.nix { };
+      };
+    };
+}