docs: add note about unsubscription

Author: daviddias

API/pubsub/README.md

@@ -15,6 +15,8 @@ pubsub API
 
 `callback` must follow `function (err, subscription) {}` where Subscription is a Node.js Stream in Object mode, emiting a `data` event for each new message on the subscribed topic.`err` is an error if the operation was not successful.
 
+`subscription` has a `.cancel` event in order to cancel the subscription.
+
 If no `callback` is passed, a [promise][] is returned.
 
 > _In the future, topic can also be type of TopicDescriptor (https://github.com/libp2p/pubsub-notes/blob/master/flooding/flooding.proto#L23). However, for now, only strings are supported._