doc: add info on project's usage of coverity

Document project's used of coverity and how
collaborators can get access.

Signed-off-by: Michael Dawson <mdawson@devrus.com>

PR-URL: https://github.com/nodejs/node/pull/40506


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>