[css-cascade] Add example per https://lists.w3.org/Archives/Public/www-style/2015Sep/0087.html

Author: fantasai

css-cascade/Overview.bs

@@ -80,6 +80,14 @@ Importing Style Sheets: the ''@import'' rule</h2>
 	(collectively, the <dfn>import conditions</dfn>)
 	state the conditions under which it applies.
 
+	<div class="example">
+		The following ''@import'' is only when the UA
+		supports ''display: flex'',
+		and is on a handheld device with a maximum screen width of 400px.
+
+		<pre>@import url("narrow.css") supports(display: flex) handheld and (max-width: 400px);</pre>
+	</div>
+
 	If a <<string>> is provided,
 	it must be interpreted as a <<url>> with the same value.