Document approach to RDF datatype boxing

[aamedina]

Problem:
Boxing RDF literals can be complex and confusing. Most of the time RDF literals are used with vocabularies declaring properties that are well-typed but occasionally you need to refer to a data value with an IRI which presents complications for Datomic attributes.

Solution:
Document the current approach which is based on creating a blank node with a single property: the IRI of the XSD datatype and the corresponding value.