add nixos development environment and guidelines

flake.nix

@@ -0,0 +1,29 @@
+{
+  description = "Strudel development environment";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.05";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            nodejs_20
+            pnpm
+            git
+          ];
+
+          shellHook = ''
+            echo "Strudel development environment loaded"
+            echo "Node.js: $(node --version)"
+            echo "pnpm: $(pnpm --version)"
+          '';
+        };
+      });
+}