Add shell flake

shells/flake.nix

@@ -0,0 +1,18 @@
+{
+  description = "Immutable NixOS development shells.";
+
+  inputs = {
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = inputs @ { self, nixpkgs, ... }:
+    inputs.flake-utils.lib.eachDefaultSystem (system:
+      let pkgs = nixpkgs.legacyPackages.${system};
+      in
+        rec {
+          devShells.default = pkgs.mkShell {
+            
+          };
+        }
+    );
+}