Improve README for site yaml files

At the moment `title` isn't required and is ignored by Transition, as `furl` is - are you trying to describe the current state or is this related to https://github.com/alphagov/transition/pull/353 and https://github.com/alphagov/bouncer/pull/84? If the latter, are we renaming them to `homepage_title` and `homepage_furl` as we discussed?