Document ikiwiki vulnerabilities

PR: 216665
Reported by: sevan
Security: CVE-2016-9645
Security: CVE-2016-10026
Security: CVE-2017-0356
Security: https://vuxml.FreeBSD.org/freebsd/5ed094a0-0150-11e7-ae1b-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/7b35a77a-0151-11e7-ae1b-002590263bf5.html