new demo slide to promote iframes/iframe backgrounds

demo.html

@@ -232,6 +232,13 @@ <h2>Background Transitions</h2>
 					<pre><code class="hljs html" style="word-wrap: break-word;">&lt;section data-background-transition="zoom"&gt;</code></pre>
 				</section>
 
+				<section data-background-iframe="https://hakim.se" data-background-interactive>
+					<div style="position: absolute; width: 40%; right: 0; box-shadow: 0 1px 4px rgba(0,0,0,0.5), 0 5px 25px rgba(0,0,0,0.2); background-color: rgba(0, 0, 0, 0.9); color: #fff; padding: 20px; font-size: 20px; text-align: left;">
+						<h2>Iframe Backgrounds</h2>
+						<p>Since reveal.js runs on the web, you can easily embed other web content. Try interacting with the page in the background.</p>
+					</div>
+				</section>
+
 				<section>
 					<h2>Pretty Code</h2>
 					<pre><code class="hljs" data-trim data-line-numbers="4,8-9">