Problem: I still haven't found any best practices for how to run HTML on
your domain without any security concerns. I feel reasonably confident
that our current security precautions mitigate any potential attacks,
but I want to be very up-front with any security mitigations that aren't
so common that they're boring and predictable. Resolves https://github.com/fraction/oasis/issues/5.
Solution: Add a document to describe our current security model and
mitigations so that we can point at it when asking friends to review the
model and ensure we don't have any holes.
