Add documentation for Response redirects

I was pretty excited to stumble over issue vlucas/bulletphp-site#13 and learn that Bullet has a built-in helper for HTTP redirects (because of course it does!), but I didn't find it mentioned anywhere in the docs. This PR adds a redirect blurb to the end of the `Response` page.

A live version is available on [my Koding VM](http://bulletdocs.gemma.kd.io/docs/response/).