Added searchbox to documentation.

This provides the entry point of the documentation search feature.

Change-Id: Iea2b707995c0e042829db509927384dcec7aaf43
diff --git a/Documentation/cmd-plugin-install.txt b/Documentation/cmd-plugin-install.txt
index 719c2bc..61663a6 100644
--- a/Documentation/cmd-plugin-install.txt
+++ b/Documentation/cmd-plugin-install.txt
@@ -71,3 +71,6 @@
 GERRIT
 ------
 Part of link:index.html[Gerrit Code Review]
+
+SEARCHBOX
+---------