[css-flexbox] Add priv/sec section.

Author: tabatkins

css-flexbox/Overview.bs

@@ -4174,3 +4174,9 @@ Clarifications</h4>
 	</ul>
 
 	A <a href="http://dev.w3.org/csswg/css3-flexbox/issues-cr-2012">Disposition of Candidate Recommendation Comments</a> is available.
+
+Privacy and Security Considerations {#priv-sec}
+===============================================
+
+	Flexbox introduces no new privacy leaks,
+	or security considerations beyond "implement it correctly".