[css-multicol-1] Adding privacy and security section

Author: rachelandrew

css-multicol-1/Overview.bs

@@ -1607,6 +1607,11 @@ This appendix is <em>informative</em>.
 	<li>Content that overflows columns is not clipped.
 </ul>
 
+<h2 class="no-num" id="priv-sec">Privacy and Security Considerations</h2>
+
+Multicol introduces no new privacy leaks,
+or security considerations beyond "implement it correctly".
+
 <h2 class=no-num id=acknowledgments>
 Acknowledgments</h2>