Simplify and update README
Simplify and update README

* Re-order so that just running Spotlight is the default
* Add details of installing grunt
* Clarify that the app can be accessed on localhost:3057, no
  need to set up a separate DNS entry

alphagov/spotlight,keithiopia/spotlight,tijmenb/spotlight,tijmenb/spotlight,alphagov/spotlight,keithiopia/spotlight,tijmenb/spotlight,alphagov/spotlight,keithiopia/spotlight