[css-writing-modes-3] Add Privacy and Security Considerations section

Author: kojiishi

css-writing-modes-3/Overview.bs

@@ -2450,6 +2450,12 @@ Full-width Characters</h5>
     can potentially affect the selection of variants for characters included in combined text runs.
     Authors are advised to use these properties with care when 'text-combine-upright' is also used.
 
+Privacy and Security Considerations {#priv-sec}
+===============================================
+
+	This specification introduces no new privacy leaks,
+	or security considerations beyond "implement it correctly".
+
 <h2 class="no-num" id="changes">Changes</h2>
 
   <h3 class="no-num" id="changes-201512">