`rel` is still required for both "linked" sub entities and "embedded" sub entities.  That section addressed the difference between a "standard" entity and an "embedded" sub-entity, however, that difference no longer exists so I thought it would make sense to remove it.

There is still the [section](https://github.com/uglymunky/siren/tree/embedded-entity#rel) that defines what a rel is and it states that it is required for sub entities.  (it just doesn't show up in the diff)  

Is there a better way to communicate the `rel` requirement?