[#618] Describe callback for sending commands in example application.

Signed-off-by: Karsten Frank <Karsten.Frank@bosch-si.com>

site/content/dev-guide/java_client_consumer.md

@@ -4,7 +4,8 @@ weight = 385
 +++
 
 To illustrate how Eclipse Hono™ can be integrated with Java code, a simple program is provided that consumes telemetry or event data 
-from the default tenant from a device. 
+from the default tenant from a device. It also includes support for sending a command to a device and receiving a response to it.
+
 It shall serve as a blueprint to integrate your existing java source code with Hono. 
 
 The code is found in the [example](https://github.com/eclipse/hono/tree/master/example) module in the package [org.eclipse.hono.vertx.example](https://github.com/eclipse/hono/tree/master/example/src/main/java/org/eclipse/hono/vertx/example).
@@ -47,6 +48,15 @@ Please note that consumers do not connect with Hono directly, but rather with an
 In the standard setup of Hono this is the [qdrouter](https://qpid.apache.org/components/dispatch-router/index.html) from the Apache Qpid project.
 In production scenarios this might be a large setup of AMQP routers, brokers, etc.
 
+### Command and Control
+
+By using a helper class provided by Hono, a callback in the application code is invoked when a downstream message was received
+that signals the device will stay connected to the protocol adapter for some time (see [Device notifications]({{< relref "concepts/device-notifications.md" >}}) for details).
+
+Inside this callback an arbitrary simple command is sent down to the device and the response is written to `System.out`.
+
+Note that for Hono 0.6 this only works for the HTTP protocol adapter.
+
 ### Encryption of communication 
 
 For the encrypted communication with Hono, the necessary truststore is already installed and used by the Hono client.