MQTT_TOPIC Macro (#76)

Added MQTT_TOPIC  macro to the list of available macros.

Signed-off-by: Zsolt Gyulai (zgyulai) <zsolt.gyulai@quest.com>

Author: mrgarris0n

doc/_admin-guide/110_Template_and_rewrite/000_Customize_message_format/004_Macros_of_syslog-ng.md

@@ -218,6 +218,11 @@ The millisecond the message was sent.
 
 Available in {{ site.product.short_name }} version 3.4 and later.
 
+## ${MQTT_TOPIC}
+
+This macro is available in syslog-ng OSE 4.7 and later versions.
+The mqtt() source sets the ${MQTT_TOPIC} name-value pair automatically for the messages it receives. This macro can be used when the name of the topic contains MQTT wildcards characters ($, +, #). For more information, see mqtt: Receiving messages from an MQTT broker.
+
 ## ${MSG}
 
 The ${MSG} macro is an alias of the ${MESSAGE} macro, using ${MSG} in