doc: improve security section of README.md

* Remove fluff text and get to the point: Report security flaws to
  security@nodejs.org. Please do not disclose security flaws publicly
  until they have been handled by the security team.
* Fix somewhat confusing paragraph that says there are no "hard
  and fast rules" but then uses _must_ in the context of a "general
  rule". Easiest solution seems to be to change _must_ to _should_.
* Minor style change (_you will_ instead of _you'll_)

PR-URL: https://github.com/nodejs/node/pull/17929


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>