message: docs: make mention of message change in changelog
diff: @@ -95,6 +95,19 @@ const subscription = topic.subscription('my-sub', {
  });
  ```
  +#### BREAKING: messages are no longer plain objects. ([#388](https://github.com/googleapis/nodejs-pubsub/pull/388))
  +
  +Messages were refactored into a [class](https://github.com/googleapis/nodejs-pubsub/blob/52305c7ee5bbc9caba1369a45ae7fdcdeba1c89b/src/subscriber.ts#L59),
  +this will only affect (some) users who treat messages like plain old objects.
  +
  +The following example is something that would have worked previously, but will
  +now throw a `TypeError` since `ack` lives on the prototype chain.
  +
  +```js
  +const m = Object.assign({}, message, customData);
  +m.ack(); // TypeError: m.ack is not a function
  +```
  +
  ### New Features
  - feat(topic): create method for publishing json ([#430](https://github.com/googleapis/nodejs-pubsub/pull/430))
  