Added crosswalk of software metadata fields.

Comparisons currently include FigShare, Zenodo, code.jsonld, and codemeta.  Needs verification and expansion to other specs/efforts.
Added example JSONLD file showing code.jsonld example.

See Arfon Smith's blog for details. http://www.arfon.org/json-ld-for-software-discovery-reuse-and-credit
Rearranged crosswalk table.
Initial codemeta.jsonld schema, and an associated instance document.

This is a work in progress, and the jsonld syntax is probably not yet correct.  The URI is certainly wrong for the @context import.  More to come...