Document available lua functions in the README.md

[samuelludwig]

Currently the keybind examples make use of available <cmd> style calls. Seeing as we now have access to vim.keymap.set, I think it would be useful to also detail the names of the functions made available by require('trouble') and what args they may take. (For instance: require('trouble').toggle() and require('trouble').toggle('workspace_diagnositcs'))