security audit => assessment

[ultrasaurus]

I refactored files as separate commits, so it's easier to review text changes.

• added process to README
• very light edits to change "SAFE examiner" to "Security Reviewer" (moved alternate suggested qualifications to: less strict requirements, if needed, for security reviewers cncf/tag-security#142)

NOTE: still WIP, just going to test out making a PR to Justin's repo