Add simple menu description to kb maps

[bdarcus]

Which-key will get support for simple menus soon.

Note: we can put the embark commands behind a submenu like so:

(setq embark-general-map `(keymap (?G . ,embark-general-map)))

I think we want to wait on doing any of this, however, until:

1. the which-key PR is merged
2. some issues are addressed in embark

Should probably add a note about non-which-key options to the README. The completing-read prompter is arguably better for this than which-key, in part because it gives you marginalia annotations, and in part because there's no problem with the UI (aside from the fact that you have to hit return after selection).

Originally posted by @bdarcus in #143 (comment)

[bdarcus]

OTOH, maybe better to just document on the wiki.