Added searchbox to documentation.

This provides the entry point of the documentation search feature.

Change-Id: Iea2b707995c0e042829db509927384dcec7aaf43
diff --git a/Documentation/config-mail.txt b/Documentation/config-mail.txt
index 3b8bffa..ca9253ee 100644
--- a/Documentation/config-mail.txt
+++ b/Documentation/config-mail.txt
@@ -211,3 +211,6 @@
 GERRIT
 ------
 Part of link:index.html[Gerrit Code Review]
+
+SEARCHBOX
+---------