reform2-keyboard: add shell.nix

Enter a shell with the AVR toolchain by running `nix-shell` in the
reform2-keyboard directory.

reform2-keyboard-fw/shell.nix

@@ -0,0 +1,8 @@
+{ pkgs ? import <nixpkgs> { } }:
+pkgs.mkShell {
+  buildInputs = with pkgs; [
+    dfu-programmer
+    pkgsCross.avr.buildPackages.gcc
+    pkgsCross.avr.buildPackages.binutils
+  ];
+}