doc: add SECURITY.md to readme.md

This adds a SECURITY.md file and links to the security document per the
request of @https://github.com/Trott at a recent SF Node meetup.

PR-URL: https://github.com/nodejs/node/pull/24031


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>