Add URL to retiring message

apps/base/templates/homepage.html

@@ -21,7 +21,7 @@
 			<div class="UI_Feature_Copy">
 				
 				{% if waffle.switch("read_only") %}
-					<h3 style="border: 1px solid red; padding: 20px; background-color: rgb(255, 238, 238);">The builder is being retired and is in read-only mode.  <a href="#">Learn More</a></h3>
+					<h3 style="border: 1px solid red; padding: 20px; background-color: rgb(255, 238, 238);">The builder is being retired and is in read-only mode.  <a href="https://blog.mozilla.org/addons/2013/12/18/add-on-builder/">Learn More</a></h3>
 				{% else %}
 					<h3>The Fastest Way to Build Add-ons</h3>