Quick start guide for MQTT service

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This is the quick start guide for the how to configure and use the MQTT service.

Basic operations of MQTT

MQTT is formed by the following operations.

1. Connect
2. Publish
3. Subscribe
4. Unsubscribe
5. Disconnect

Before performing this operations, must call mqtt_init function.

Connect

1. Connect operation is required for the other operations.
2. Connect operation is consist of following 2 functions.

   * mqtt_connect : This function preforms the connect to MQTT broker server.
   * mqtt_connect_broker : This function preforms the authenticate and configure options to the MQTT broker server.
   * 

3. After mqtt_connect operation has completed MQTT_CALLBACK_SOCK_CONNECTED event will produced. It means server was found and connected.
4. After that you should call mqtt_connect_broker function and configure this MQTT connection. After this operation finished, MQTT_CALLBACK_CONNECTED event will produced.

Publish

1. Publish operation perform the send message to all of subscribers that share the same topic.
2. Publish operation is consist of following function.

   * mqtt_publish : This function preforms the publish message to the subscribers
   * 

Subscribe

1. Subscribe operation is used to get the message on a particular topic when it is generated.
2. Subscribe operation is consist of following function.

   * mqtt_subscribe : This function preforms the subscription of the topic.
   * 

Disconnect

1. Disconnect operation is used to disconnect from the MQTT broker server.
2. Disconnect operation is consist of following function.

   * mqtt_unsubscribe : This function preforms the unsubscribe topic.
   * 

Unsubscribe

1. Unsubscribe operation is used to cancel a subscription of the specific topic.
2. Unsubscribe operation is consist of following function.

   * mqtt_disconnect : This function preforms the disconnection from the broker server.
   *