Specify that contexts should be in JSON-LD

[anatoly-scherbakov]

Here: https://w3c.github.io/json-ld-syntax/#loading-documents

[BigBlueHat]

It may just be that YAML-LD needs it's own "Loading Documents" section to describe what SHOULD/MUST be done particularly as regards remote contexts.

[TallTed]

It may just be that YAML-LD needs it's own "Loading Documents" section

A good start.