[Documentation] Update README with the more powerful chat command based interface

The README talks a lot about a GUI/Formspec, I took a different approach as the formspec/gui is considerably harder to develop/maintain.

Instead I'm using a chat command based interface, which provides some difficulty in usage but is considerably easier to develop/maintain.

Currently as of [7646ee3a] the codebase is actually almost done, we have a few features which I'd like to implement like percent based regeneration rather than the current only time based regeneration.

All the commands include help, which probably will save a lot of guessing.