needs documentation

[ericprud]

Extract docs from:

• Specification: ShEx Semantics
• Example: ShExJ

[cmungall]

Maybe a section on motivation would help too - on arriving at this repo without any context my immediate thought is "isn't this what json schema is for?"

[ericprud]

There's a bit on the grammar in the top of the ShEx spec: https://shex.io/shex-semantics/#json-grammar .
Ideally, this would be in it's own doc (and way better) and referenced from future versions of the ShEx spec.