Add a simple shell.nix to bring in qmk

Solves: https://github.com/qmk/qmk_userspace/issues/8

.envrc

@@ -0,0 +1 @@
+use nix

shell.nix

@@ -0,0 +1,10 @@
+let
+  nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/nixos-24.05";
+  pkgs = import nixpkgs { config = {}; overlays = []; };
+in
+
+pkgs.mkShellNoCC {
+  packages = with pkgs; [
+      qmk
+  ];
+}