Add nix flake for dev shell

flake.nix

@@ -0,0 +1,24 @@
+{
+  description = "Terminal session recorder";
+
+  inputs = {
+    nixpkgs.url = github:nixos/nixpkgs/nixpkgs-unstable;
+    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 {
+            nativeBuildInputs = with pkgs; [
+              rustup
+            ] ++ (lib.optionals stdenv.isDarwin [
+	      libiconv
+	      darwin.apple_sdk.frameworks.Foundation
+	    ]);
+          };
+        }
+      );
+}