Add disclaimer about the new changes and add link to the old version

Change-Id: Ib75ea5c6560fa874b3eb3f605ca114a9d2202fcb
diff --git a/named_data_theme/layout.html b/named_data_theme/layout.html
index 65681b8..8e1de4c 100644
--- a/named_data_theme/layout.html
+++ b/named_data_theme/layout.html
@@ -19,6 +19,7 @@
              <!--top menu-->
              <div class="nine columns" id="menu_container" >
                <h1><a href="{{ pathto(master_doc) }}">{{ shorttitle|e }}</a></h1>
+               <h2>Note that this is the version of NDN packet specification in progress.  Not all changes are finalized and supported by the existing NDN codebase</h2>
              </div>
         </div>
     </div><!--header container end-->