Add notes in Book for handling sub-Types

[jmckenna]

from @pbuttigieg :

• The partners are sharing Typed records.
• Within these records, there are often other Types embedded to fill out value fields for properties.
  • These Typed values are not the primary type shared, and often are not given as much attention / curation
  • If the partner wanted to share a fully curated Type record as a value for a property in another record, they should use the @id of another record with full metadata to link off to that record

related to https://github.com/iodepo/oih-ui/issues/85