Add security considerations about fingerprint usability
Note that I couldn't find good references about fingerprint usability to include explicitly in the draft.
The most promising study design that i recall (from 2014, see https://github.com/tomrittervg/crypto-usability-study) doesn't seem to have been completed.
However, the design team seems to agree that manual fingerprint comparison by humans is dubious, so hopefully this reflects that consensus.
Closes: #44 (closed)
Note that this is mutually exclusive with !103 (closed)