Add flake.nix for easy building

flake.nix

@@ -0,0 +1,14 @@
+{
+  description = "A high-performance Wayland screen locker written in Rust";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs }: let
+    system = builtins.currentSystem;
+    pkgs = import nixpkgs { inherit system; };
+  in {
+    packages.${system}.rustlock = pkgs.callPackage ./default.nix {};
+  };
+}