add link to serif theme in default presentation

index.html

@@ -163,6 +163,7 @@
 						<a href="http://lab.hakim.se/reveal-js/?theme=sky">Sky</a> -
 						<a href="http://lab.hakim.se/reveal-js/?theme=beige">Beige</a> -
 						<a href="http://lab.hakim.se/reveal-js/?theme=simple">Simple</a> -
+						<a href="http://lab.hakim.se/reveal-js/?theme=serif">Serif</a> -
 						<a href="http://lab.hakim.se/reveal-js/">Default</a>
 					</p>
 					<p>