libraries\src\Document\JsonapiDocument.php missing addIncluded function

[fuscage]

Hello :-)
I think it missing "included" field in the jsonapidocument :
see the exemple at https://jsonapi.org/
https://cloud.githubusercontent.com/assets/1140906/19152663/613af900-8bd3-11e6-8b28-69321e2579f5.png

That's will be great to add this possibilty for the futur

Thanks :)