Add SQL++ Tutorial reference

Change-Id: I7a27a5576ae21b6fda440bfc281d21e626385156
Reviewed-on: https://asterix-gerrit.ics.uci.edu/3019
Reviewed-by: Ian Maxon <imaxon@apache.org>
diff --git a/_includes/nav_header.html b/_includes/nav_header.html
index 498cda3..e056d13 100644
--- a/_includes/nav_header.html
+++ b/_includes/nav_header.html
@@ -28,6 +28,9 @@
                   <li class="divider"></li>
                   <li class="dropdown-header">Snapshot release</li>
                   <li><a href="{{ site.UNSTABLE_DOCS_BASE_URL }}index.html">Documentation for {{ site.UNSTABLE_VERSION }}  <i class="fa fa-external-link fa-sm"></i></a></li>
+                  <li class="divider"></li>
+                  <li class="dropdown-header">Tutorial for SQL Users</li>
+                  <li><a href="{{ site.url }}files/SQL_Book.pdf">Don Chamberlin's SQL++ for SQL Users<i class="fa fa-external-link fa-sm"></i></a></li>
                 </ul>
               </li>
               <li><a href="{{ site.url }}about.html">About</a></li>