[Release notes] Migrate from Hibernate ORM 5.0 to Hibernate ORM 5.1

The section Migrate from Hibernate ORM 5.0 to Hibernate ORM 5.1 contains a link to the migration guide. However, the migration guide will not be released with Alpha, so that link won't work. We need to take some of the contents of the relevant chapter in migration guide and copy it into the release notes.