Minor changes to README, update gitmodules.

Add:
	gnome_terminal/
		-add temporarily final version of my `gnome-terminal` configuration;
		seeing as I don't expect extensive changes to it in the near future,
		will be flipping the directory's `assume-unchanged` bit.

Rem:
	.gitmodules
		-remove now inexistent "zsh-syntax-highlighting" submodule.