Document standard threat models that Enarx addresses (RFC)

[MikeCamel]

Description
We don't have a well-documented set of threat models. These are important to make explicit, for:
a) designers and developers of Enarx;
b) users of Enarx;
c) auditors of Enarx;
d) auditors of deployments using Enarx.

Where available, we should link to threat models for component/dependent parts, such as SGX, as (following the "weakest link" model), Enarx cannot expect to address threat models for which component/dependent parts do not cover.

[MikeCamel]

See also enarx/enarx#221.