Draft TAG Explainer

[markafoltz]

This issue tracks a followup item from TPAC 2018 [1], namely, to write an explainer document to help TAG or other outside groups understand the problem we're trying to solve and the outline of our solution.

Explainer explainer: https://github.com/webmachinelearning/webnn/blob/master/explainer.md or https://github.com/w3ctag/w3ctag.github.io/blob/master/explainers.md

[1] https://www.w3.org/2018/10/26-webscreens-minutes.html#x13

Part of this is also finishing the document on custom schemes and interoperability (Issues #93 and #94).