chore: update readme code example to new API. (#382)

* Update readme code example to new API.

* Change readme code example to show how to use with libp2p.

README.md

@@ -35,18 +35,21 @@ Gossipsub is an implementation of pubsub based on meshsub and floodsub. You can
 ## Usage
 
 ```javascript
-const Gossipsub = require('@chainsafe/libp2p-gossipsub')
+import { gossipsub } from '@chainsafe/libp2p-gossipsub'
 
-const gsub = new Gossipsub(libp2p, options)
 
-await gsub.start()
+const libp2p = await createLibp2p({
+  // ...
+  pubsub: gossipsub(options)
+});
 
-gsub.on('fruit', (data) => {
-  console.log(data)
+libp2p.pubsub.addEventListener('message', (message) => {
+  console.log(`${message.detail.topic}:`, new TextDecoder().decode(message.detail.data))
 })
-gsub.subscribe('fruit')
 
-gsub.publish('fruit', new TextEncoder().encode('banana'))
+libp2p.pubsub.subscribe('fruit')
+
+libp2p.pubsub.publish('fruit', new TextEncoder().encode('banana'))
 ```
 
 ## API
@@ -55,7 +58,7 @@ gsub.publish('fruit', new TextEncoder().encode('banana'))
 
 ```js
 const options = {…}
-const gossipsub = new Gossipsub(libp2p, options)
+const gossipsub = gossipsub(options)(libp2p)
 ```
 
 Options is an optional object with the following key-value pairs: