Update README.md

Author: Tuyen

README.md

@@ -67,6 +67,10 @@ Options is an optional object with the following key-value pairs:
 
 * **`fallbackToFloodsub`**: boolean identifying whether the node should fallback to the floodsub protocol, if another connecting peer does not support gossipsub (defaults to **true**).
 * **`emitSelf`**: boolean identifying whether the node should emit to self on publish, in the event of the topic being subscribed (defaults to **false**).
+* **`msgIdFn`**: a function defining the message id given a message, this is optional. For example:
+```js
+const msgIdFn = (message) => Buffer.from(hash(message.data)).toString("base64")
+```
 
 For the remaining API, see https://github.com/libp2p/js-libp2p-pubsub