doc: add prompt screenshots to README
doc: add prompt screenshots to README

There is more that could be said about the zsh set-up here, but I am too
lazy for now.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent