[TASK] Add security section to README

Resolves: #89481
Releases: master, 9.5, 8.7
Change-Id: I91406b5678f11b6fc0ec433e58325dd81dbfc17a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62075


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Torben Hansen <derhansen@gmail.com>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Torben Hansen <derhansen@gmail.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>