Added link to upgrade from source to Omnibus

source/installation/index.html

@@ -40,9 +40,8 @@ extra_css:
       <div class="method">
         <h2>Update your GitLab instance</h2>
         <ul>
-          <li>
-            <p>
-              <a href="/update">Update options</a> for GitLab Community Edition and Enterprise Edition.
+            <li><p><a href="/update">Update options</a> for GitLab Community Edition and Enterprise Edition</p></li>
+            <li><p><a href="https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/doc/update.md#upgrading-from-a-non-omnibus-installation-to-an-omnibus-installation">Upgrade</a> from a source installation to an Omnibus installation 
             </p>
           </li>
         </ul>
@@ -117,4 +116,4 @@ extra_css:
     </div> <!-- col-md-12 -->
   </div> <!-- wrapper-container -->
-</div> <!-- content -->
+</div> <!-- content -->
\ No newline at end of file