Make Classroom + Chat Command Builder dependencies clear

I `git clone`d CTW into my `.minetest/games/` directory to install it, and when I ran Minetest had to install Ruben Wardy's Classroom and Chat Command Builder mods (through the Minetest UI). The game did not make this clear for a non-technical user, and instead emitted Lua errors saying the dependencies were not present.

You should probably either mention this as installation instructions in the README, or include the dependencies in some way. They are not automatically installed in Minetest 5.8.0.