[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 thought that the `{% include %}` tag was deprecated in favor of the `include()` function, but I don't know if this is the case for the Symfony documentation. So, should we replace this by the following?

```jinja
{{ include('AcmeTaskBundle:Task:prototypeTask.html.twig', { 'form': form.task.vars.form }) }}
```