update CHANGELOG with libxml2 USN info

Author: flavorjones

CHANGELOG.rdoc

@@ -9,6 +9,10 @@ See these libxml2 email posts for more:
 * https://mail.gnome.org/archives/xml/2015-November/msg00012.html
 * https://mail.gnome.org/archives/xml/2016-May/msg00023.html
 
+For a more detailed analysis, you may care to read Canonical's take on these security issues:
+
+* http://www.ubuntu.com/usn/usn-2994-1
+
 
 [MRI] Bundled libxslt is upgraded to 1.1.29, which fixes a security issue as well as many long-known outstanding bugs, some features, some portability improvements, and general cleanup.