Please update Usage docs when introducing new fields

[brettforbes]

Hi, Just a heads-up

I found a new field/property in the latest version, which is not reported in the USAGE.md file. I caught it because my input python classes (extensions to the OASIS Stix2 Python library) threw an exception while trying to parse the object, before translating into my schema language. The property is "x_mitre_network_requirements": false, the new class property is ('x_mitre_network_requirements', BooleanProperty()), (note i didn't set it as false by default)

I notice the USAGE.md file hasn't been updated for eight months. Might be a good time to amend the property table for Techniques and Subtechniques