Add docs on what exact validation vtvalidate does

[springmeyer]

vtvalidate currently will throw (or provide an error message) if vtzero (the decoder used in vtvalidate) cannot decode a message. This needs to be documented so users of vtvalidate do not need to hunt into the vtzero code to know exactly what validation is being performed.

This doc should be a living doc, per version, as we (potentially) add more validation over time.
refs:

• Geometry handling: strict vs. non-strict mode vtzero#23
• https://github.com/mapbox/vtzero/blob/master/examples/vtzero-check.cpp

[GretaCB]

First iteration over at #26