JsonSchema export documentation

[dredozubov]

@sboosali proposed to port json-schema examples to schematic and document how the json-schema export works, so I'm separating this as an isolated issue.

As you can see, this library is very conservative in regard of json-schema version:
https://github.com/typeable/schematic/blob/master/src/Data/Schematic/JsonSchema.hs#L25

I haven't checked if it's compatible with newer drafts, but it's highly possible because schematic uses a strict subset of json-schema.