[RN] Add Hibernate ORM 5.1 features directly in release notes

It was requested by gbadner@redhat.com to put the Hibernate ORM 5.1 features directly in the release notes instead of linking to sebersole's blog (we did something similar for the Hibernate Validator 5.3.x). Also to exclude the "@Embeddables" feature, as it is not ready for production.

The migration guide also currently lists these features, so once they are in the release notes, make an update to the migration guide to point to the release notes.