Add link to JSON-LD spec in README

Specifically, the astool README.

Also, elaborate on the OWL definition of the ActivityStreams vocabulary:

Non-standard special JSON-LD/RDF property named "@wtf_without_property" in the definition because I couldn't find anything in OWL or any other RDF schema that defines a "don't-have-this-type-inherit-this-specific-property-when-it-extends-from-a-parent-type-with-this-property" semantic meaning
This property is used for the 'object' property not present on IntransitiveActivity and 'items' property for OrderedCollections. The former is defined in the spec but the latter is very much an opinionated interpretation of the spec.

go-fed / activity