Update README.md with changes to 'publish message with metadata'

Added changed suggested to align with PEP8 and make it a bit clearer

Author: cameron-michie

README.md

@@ -102,14 +102,14 @@ logger.addHandler(logging.StreamHandler())
 await channel.publish('event', 'message')
 ```
 
-### Publishing a message to a channel including metadata
-
+If you need to add metadata when publishing a message, you can use the `Message` constructor to create a message with custom fields:
 ```python
 from ably.types.message import Message
-messageObject = Message(name="messagename",
-                        data="payload",
-                        extras={"headers": {"metadataKey": "metadataValue"}})
-await channel.publish(messageObject)
+
+message_object = Message(name="message_name",
+                         data="payload",
+                         extras={"headers": {"metadata_key": "metadata_value"}})
+await channel.publish(message_object)
 ```
 
 ### Querying the History