[WIP] Add section for rendering a custom prototype for collections

This PR adds an section to the prototype cookbook-article to describe how to render a custom prototype for dealing with collections.
I don't know if the right word is "deprecated", but definitely the `include()` function is preferred. @fabpot [gave](https://github.com/fabpot/Twig/pull/926#issuecomment-11195473) these reasons:

>  Using a function allows you to do whatever you want with the output (which is not possible with a tag), like a simple:
>
> {{ set content = render('some_template') }}
>
> Also, semantically, using a tag to output some content is less correct.