Request-based README

![chicago.gif](http://i.imgur.com/Sii8YKk.gif)

README for the Broker/Renderer request-based example, see following PRs for context:

* https://github.com/BBC-News/alephant/pull/53
* https://github.com/BBC-News/alephant/pull/54
I'd remove any mention of `poll` as it's not polling, it's just hitting the predefined endpoint when something hits it to renderer a template.