add alert about v3 to v2 page

2/index.html

@@ -80,6 +80,12 @@
 	</div>
 
 	<div class="hero-unit">
+
+		<div class="alert alert-danger">
+		  <p><h4 class="alert-heading">Have you upgraded?</h4></p>
+		  <p><a href="../">Visit the main page</a> to get the latest Bootstrap themes.</p>
+		</div>
+
 		<h1>Bootswatch</h1>
 		<p id="tagline">&nbsp;</p>