make initial-letter intro non-normative

Author: dauwhe

css-inline-3/Overview.bs

@@ -292,6 +292,8 @@ Initial Letters</h2>
 <!--<a name="DropInitial"></a>-->
 An Introduction to Initial Letters</h3>
 
+	<em>This section is non-normative.</em>
+
 	Large, decorative letters have been used to start new sections of text since before the invention of printing.
 	In fact, their use predates lowercase letters entirely.