doc: add process for handling premature disclosure

Add process for handling premature disclosure of
a security vulnerability in the public repos.

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

PR-URL: https://github.com/nodejs/node/pull/36155


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Mary Marchini <oss@mmarchini.me>
Reviewed-By: Rich Trott <rtrott@gmail.com>