Add reference to CTI JSON Schemas

[ikiril01]

We should add references to the CTI JSON STIX 2 schemas for our JSON schemas, especially for the the observable-objects and individual STIX Observable Objects.

https://github.com/oasis-open/cti-stix2-json-schemas

[gtback]

@ikiril01 Are you planning to use this repo for the MAEC 5 schemas? I noticed there are already branches.

If you do, can I suggest just switching the default branch to the 5.0 branch, and leaving master untouched, or at least saving the current master branch as a 4.x branch?

Regarding this specific issue, I don't know if you can reference JSON schemas by URL (probably), or if you need to somehow use a git submodule to include them in this repo. Either way, I can help out if you need me to.