Advocate using yeoman generator as recommended method creating a hubot instance

This one's a shot in the dark. I'm not operating this way myself right now, but thought I'd toss it out there for discussion :)

This would be instead of using `hubot --create` command. Might help combat the inception-like confusion that people have had historically, by creating a cleaner separation of "hubot the bot" and "hubot the tool to generate the bot"

## TODO

- [x] Update ["Getting Starting" docs](https://github.com/github/hubot/tree/master/docs#getting-started-with-hubot)
- [x] Remove [baked-in creator code](https://github.com/github/hubot/blob/master/src/creator.coffee)
- [ ] Transfer [generator-hubot-bot project](https://github.com/patcon/generator-hubot-bot) ownership to github org
- [x] Sensible helper text
- [x] Update [`CONTRIBUTING.md`](https://github.com/github/hubot/blob/master/CONTRIBUTING.md)

cc: @desmondmorris