Add Extension Build Integration Doc

Change-Id: I8329d7470016894da7a4c26f2942af6e19727d8d
Reviewed-on: https://asterix-gerrit.ics.uci.edu/1262
Reviewed-by: Till Westmann <tillw@apache.org>
diff --git a/_includes/nav_header.html b/_includes/nav_header.html
index 7ca1883..498cda3 100644
--- a/_includes/nav_header.html
+++ b/_includes/nav_header.html
@@ -41,7 +41,7 @@
                 </ul>
               </li>
               <li class="dropdown">
-                <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-expanded="false">Contributing <span class="caret"></span></a>
+                <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-expanded="false">Development<span class="caret"></span></a>
                 <ul class="dropdown-menu" role="menu">
                   <li class="dropdown-header">For New Contributors</li>
                   <li><a href="{{ site.url }}dev-setup.html">Development Setup</a></li>
@@ -49,6 +49,8 @@
                   <li class="dropdown-header">For Committers</li>
                   <li><a href="{{ site.url }}pushing.html">Pushing changes</a></li>
                   <li><a href="{{ site.url }}site.html">Editing this site</a></li>
+                  <li class="dropdown-header">Extensions</li>
+                  <li><a href="{{ site.url }}extension.html">Extending AsterixDB</a></li>
                 </ul>
               </li>
             </ul>