Update README with basic installation instructions.
Update README with basic installation instructions.

Both @workwithgreat and @phae had to get the app running locally and we
realised that there weren't any installation instructions written down.

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