Add sanitizing content section

Closes #108.

index.html

@@ -1780,6 +1780,15 @@ Location: https://dustycloud.org/likes/345
           sort of spam filter.
         </p>
       </section>
+
+      <section id="security-sanitizing-content">
+        <h2>Sanitizing Content</h2>
+        <p>
+          Any activity field being rendered for browsers (or other rich
+          text enabled applications) should take care to sanitize fields
+          containing markup to prevent cross site scripting attacks.
+        </p>
+      </section>
     </section>
 
     <section class="appendix informative" id="acknowledgements">