Add details about Qt Project security policy. The SECURITY.md file
is now required by the yocto-check-layer script.
Pick-to: 6.9 6.8
Change-Id: Icbcd63bb15c0d106b1bde4c2b9c43aebe1031797
Reviewed-by: Mikko Gronoff <mikko.gronoff@qt.io>
Reviewed-by: Inkamari Harjula <inkamari.harjula@qt.io>
Reviewed-by: Ari Parkkila <ari.parkkila@qt.io>
