[css-display] Mark Appendix B as non-normative.

Author: tabatkins

css-display/Overview.bs

@@ -912,6 +912,8 @@ Appendix A: Glossary</h2>
 <h2 class="no-num non-normative" id="unbox">
 Appendix B: Effects of ''display: contents'' on Unusual Elements</h2>
 
+	<em>This section is (currently) non-normative.</em>
+
 	Some elements aren't rendered purely by CSS box concepts;
 	for example, replaced elements (such as <{img}>),
 	many form controls (such as <{input}>),