Add NIX flake package

flake.nix

@@ -0,0 +1,25 @@
+{
+  description = "Activate Linux watermark for wayland";
+
+  inputs.utils.url = "github:numtide/flake-utils";
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs";
+
+  outputs = { self, nixpkgs, utils }:
+    utils.lib.eachSystem [ "x86_64-linux" "aarch64-linux" ] (system:
+      let pkgs = nixpkgs.legacyPackages.${system};
+      in {
+        packages.activate-linux = pkgs.stdenv.mkDerivation {
+          pname = "activate-linux";
+          version = "0.0.1";
+          src = ./.;
+          nativeBuildInputs = with pkgs; [ pkg-config ];
+          buildInputs = with pkgs; [ gtk3 gtk-layer-shell ];
+          installPhase = ''
+            mkdir -p $out/bin
+            cp bin/activate-linux $out/bin/activate-linux
+          '';
+        };
+
+        defaultPackage = self.packages.${system}.activate-linux;
+      });
+}