Document "pulling" data into components using ref blocks

Use of `ractive.set('foo')` where `foo` is a parent data value and `foo` is not used in the template or passed in explicitly won't work. But if you put `{{#foo}}{{/}}` in the template, ractive knows about it and subsequent `set` or `get` operations work.

Probably we could make them (get and set) smarter, but I finally got my head around this and it's worth documenting.
