Generate a more helpful readme

(#16)

Some of the writing borrowed from [the forthcoming Yeoman introduction walkthrough I'm writing](http://mebutyou.com/#6-10-2013-building-apps-with-the-yeoman-workflow).

Since a generator doesn't necessarily use Grunt or Bower, I didn't go as far as walking through those tools. They were given a shout-out and a link, however.