Added searchbox to documentation.

This provides the entry point of the documentation search feature.

Change-Id: Iea2b707995c0e042829db509927384dcec7aaf43

Documentation/json.txt

diff --git a/Documentation/json.txt b/Documentation/json.txt
index 9278cde..73dbd92 100644
--- a/Documentation/json.txt
+++ b/Documentation/json.txt
@@ -276,3 +276,6 @@
 GERRIT
 ------
 Part of link:index.html[Gerrit Code Review]
+
+SEARCHBOX
+---------