Add document for message structure
diff --git a/src/main/resources/Documentation/message.md b/src/main/resources/Documentation/message.md
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+Message structure
+======================
+
+This plugin publish message like the below format to RabbitMQ.
+
+*Italic* is key name in `rabbitmq.config`.
+
+**Bold** is literal.
+
+Properties
+=======================
+
+| name | value |
+|:-----------------|:---------------------------|
+| app_id | **gerrit** |
+| priority | *message.priority* |
+| delivery_mode | *message.deliveryMode* |
+| headers | <See Header section> |
+| content_encoding | **UTF-8** |
+| content_type | **application/json** |
+
+
+Headers
+=======================
+
+| name | value |
+|:-----------------|:--------------------------------------------|
+| gerrit-name | *gerrit.name* |
+| gerrit-host | *gerrit.hostname* |
+| gerrit-scheme | *gerrit.scheme* |
+| gerrit-port | *gerrit.port* |
+| gerrit-front-url | *gerrit.canonicalWebUrl* in `gerrit.config` |
+| gerrit-version | gerrit version |
+
+
+Payload
+=======================
+
+Payload is JSON string. (same gerrit-events)
