Add shell.nix for NixOS users

shell.nix

+with import <nixpkgs> {};
+stdenv.mkDerivation {
+  name = "redox";
+
+  hardeningDisable = [ "all" ];
+
+  nativeBuildInputs = [ gnumake cmake nasm pkgconfig gcc automake autoconf bison gperf qemu ];
+  buildInputs = [ openssl gettext libtool flex libpng perl perlPackages.HTMLParser ];
+}