[mediacapture-record] Pull Request: more "Privacy and Security Considerations" text

yellowdoge has just submitted a new pull request for https://github.com/w3c/mediacapture-record:

== more "Privacy and Security Considerations" text ==
(partially) addressing #141 -- arguably a bit delayed, but I didn't know where or how to start, so I read 
https://w3ctag.github.io/security-questionnaire/
https://www.w3.org/TR/fingerprinting-guidance/
and https://www.torproject.org/projects/torbrowser/design/#fingerprinting-linkability

@npdoty, @dontcallmedom PTAL

See https://github.com/w3c/mediacapture-record/pull/165