doc: add Roadmap link to header

Update to match spdk.io.

Change-Id: I05ef119d9756a5af77d22c5ca0db438b549dbbf8
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>

doc/header.html

@@ -78,6 +78,13 @@
                 </a>
               </li>
 
+              <li role="presentation">
+                <a href="http://www.spdk.io/roadmap/">
+                  <i class="glyphicon glyphicon-map-marker"></i>
+                  <span class="box-name">Roadmap</span>
+                </a>
+              </li>
+
             </ul>
           </div>
         </section>