Tidy up README
Tidy up README

The README now explains what "whitehall" is.

Additionally, I've formatted the README so that it hard-wraps at 80 columns.

askl56/whitehall,hotvulcan/whitehall,ggoral/whitehall,YOTOV-LIMITED/whitehall,alphagov/whitehall,robinwhittleton/whitehall,robinwhittleton/whitehall,ggoral/whitehall,alphagov/whitehall,YOTOV-LIMITED/whitehall,askl56/whitehall,ggoral/whitehall,YOTOV-LIMITED/whitehall,askl56/whitehall,robinwhittleton/whitehall,hotvulcan/whitehall,hotvulcan/whitehall,alphagov/whitehall,YOTOV-LIMITED/whitehall,ggoral/whitehall,askl56/whitehall,alphagov/whitehall,hotvulcan/whitehall,robinwhittleton/whitehall