add nix flake

flake.nix

@@ -0,0 +1,24 @@
+{
+  description = "Rush - endless runner for Playdate";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    playdate-sdk = {
+      url = "github:RegularTetragon/playdate-sdk-flake";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = { self, nixpkgs, playdate-sdk }:
+    let system = "x86_64-linux";
+        pkgs = nixpkgs.legacyPackages.${system};
+        playdate-sdk-pkg = playdate-sdk.packages.${system}.default;
+    in {
+      devShell.${system} = with pkgs;
+        mkShell {
+          nativeBuildInputs = [
+            playdate-sdk-pkg
+          ];
+        };
+  };
+}