Add property aims to Spec

[nicholascar]

The Specification doesn't always tell you why a property is required or what it's value should be, if present.

e.g.: rdfs:isDefinedBy - this property is included to ensure the origin of a Concept, where it is defines, is known, as opposed to where it is present (skos:inScheme/skos:topConceptOf). This is important when Concepts are used in multiple vocabs