Move threat model doc to GitHub

Move threat model doc to GitHub
The easy way to handle this one would be to add the following at the end of the second bullet point: "Keep in mind that these actions require the attacker to have different access levels on the system; an attacker with root on the system has vastly different capabilities than an attacker who has access to the *tor* user but no way to escalate." Thoughts? I can elaborate on this a bit more if need be.