Updated header and added link to the ES Ref.

debadair · debadair · commit eb0004640922 · 2018-06-25T15:51:08.000-07:00
diff --git a/page_header.html b/page_header.html
@@ -1 +1,4 @@
-We are working on updating this book for the latest version. Some content might be out of date.
+This information may not apply to the latest version of Elasticsearch.
+For the most up to date information, see the current version of the
+<a href="https://www.elastic.co/guide/en/elasticsearch/reference/current/index.html">
+Elasticsearch Reference</a>.
