README should list time of audit (or version audited)

[64]

Otherwise 2 years later we have a misleading notice saying 'X is safe' when a new PR added 50 extra unsafe blocks.

[Lokathor]

Let's not dance around the real thing we need to do here: Everyone gets a cargo crev ID and we can list their IDs / URLs in a repo here and people can sign up and use the tool. It already accounts for version differences when adding up reviews.

[Lokathor]

(And also update the README file too)