Add description of various timer related vulnerabilities to the security and privacy sections

[yoavweiss]

In #79 (comment) we outlined a bunch of timing related attacks, required granularity for them and potential mitigations. It might make sense to add relevant ones to the Security & Privacy sections, as examples implementers should be aware of.

[jyasskin]

One particular attack: we should cite Fantastic Timers (https://gruss.cc/files/fantastictimers.pdf) and mention edge-based timing as a thing implementations should consider mitigating.