add threat model draft

[eriktaubeneck]

+@marianapr

An initial draft of the threat model document for review by the community group.

Given this came together over a various vacations and other leaves, I'll let other contributor tag themselves in the comments.

[seanturner]

Thanks for pulling this together. We need to start somewhere and sometime.

[AramZS]

Looks good, thanks!

[AramZS]

This document is a work in progress and not a final document of the group. We are merging in here to make additional contributions easier to manage.