[WIP] new cookbook article on using empty data for form classes

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes (symfony/symfony#1140)
| Applies to    | all
| Fixed tickets | #998

I'd like some feedback on my cookbook article for both accuracy and formatting. I am working on linking this cookbook article from the form class section of the documentation, as well as the two form types (choice and collection) that use the empty_data option. Those are both relevant, correct?