[WIP] Add REST API cookbook entry

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all (or 2.3+)
| Fixed tickets | 

Following [this thread](https://groups.google.com/forum/#!topic/resting-with-symfony/-09AA7E_PYs) on the resting-with-symfony ML, 
we think adding a cookbook entry about REST API could be a good thing. FOSRestBundle is not the only response here and I think it's important to get some basics skills before using such a bundle.

At the moment I only have a quick plan,
but we need to choose what we should expose in this documentation for the update / create actions.

- using the form component to update the resource via "application/x-www-form-urlencoded"?
- using the serializer to parse a payload?
- using something like the [RestBundle BodyListener](https://github.com/FriendsOfSymfony/FOSRestBundle/blob/master/EventListener/BodyListener.php) and the [JsonToFormDecoder](https://github.com/FriendsOfSymfony/FOSRestBundle/blob/master/Decoder/JsonToFormDecoder.php)?

Also we need to draw the line somewhere about RESTful. I do not think we should tell in this cookbook how to implement everything (hypermedia and such).

Please ping me if you have time to help, I will give you write access to the branch directly.
i like this approach that is not tied to FOSRestBundle but that goes in detail on the inner components