Document Joomla! security advisories since 3.4.6 was released.

While here, update entry for 3.4.6 with final advisory information from
JSST page.

A big thanks to Larry Rosenman for reporting the open issues and getting the
port up to date.

PR: 215058
Reported by: Larry Rosenman <ler@lerctr.org>
Security: CVE-2016-8869
Security: CVE-2016-8870
Security: CVE-2016-9081
Security: CVE-2016-9836
Security: CVE-2016-9837
Security: CVE-2016-9838
Security: https://vuxml.FreeBSD.org/freebsd/624b45c0-c7f3-11e6-ae1b-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/a27d234a-c7f2-11e6-ae1b-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/c0ef061a-c7f0-11e6-ae1b-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/f0806cad-c7f1-11e6-ae1b-002590263bf5.html