Move threat model doc to GitHub

Move threat model doc to GitHub
The primary goal of Tails is not to leave traces on the machine that it's run on, but the device Tails was install on (e.g. a USB stick) will have traces of both the operating system and the persistent drive. While we do not encourage sources to use persistent volumes, we do not say anything about how the source should handle the information that he or she is about to submit. On the off-chance that the source does decide to store this information on a Tails persistent drive, I figured it would be good to include it.