[css-fonts-4] add another multi-format example

Author: svgeesus

css-fonts-4/Overview.bs

@@ -2144,6 +2144,15 @@ for user agents which don't support a particular font technology.
 	</pre>
 	</div>
 
+	<div class="example">
+		In this example, if woff2 is not supported,
+		an unsupported and fictitious "zebra" format is skipped
+		and the opentype font is downloaded:
+		<pre>src: url(ideal.woff2) format("woff2"),
+		url(unsupported.zeb) format("zebra"),
+		url(basic.ttf) format("opentype");</pre>
+	</div>
+
 	<div class="example">Here an individual instance in a collection is loaded,
 	or if collections are not supported, download an individual font instead.
 	<pre>