[css-extensions] Write a simple example in the block syntax.

Author: tabatkins

css-extensions/Overview.bs

@@ -58,6 +58,16 @@ Extension Names</h2>
 	in the above definitions.
 
 	<div class='issue'>
+		Simple things are easy:
+
+		<pre>
+		@custom-selector :--heading {
+			expansion: h1, h2, h3, h4, h5, h6;
+		}
+		</pre>
+
+		More complicated things are possible:
+
 		<pre>
 		// Arguments are specified with $foo.
 		// An arg before the pseudo-class captures the rest of the compound selector.