docs: auto relay example (#795)

* chore: auto relay example

* chore: update examples to use process arguments

* chore: add test setup for node tests and test for auto-relay

* chore: apply suggestions from code review

* chore: do not use promise for multiaddrs event on example

examples/auto-relay/listener.js

@@ -0,0 +1,47 @@
+'use strict'
+
+const Libp2p = require('libp2p')
+const Websockets = require('libp2p-websockets')
+const { NOISE } = require('libp2p-noise')
+const MPLEX = require('libp2p-mplex')
+
+async function main () {
+  const relayAddr = process.argv[2]
+  if (!relayAddr) {
+    throw new Error('the relay address needs to be specified as a parameter')
+  }
+
+  const node = await Libp2p.create({
+    modules: {
+      transport: [Websockets],
+      connEncryption: [NOISE],
+      streamMuxer: [MPLEX]
+    },
+    config: {
+      relay: {
+        enabled: true,
+        autoRelay: {
+          enabled: true,
+          maxListeners: 2
+        }
+      }
+    }
+  })
+
+  await node.start()
+  console.log(`Node started with id ${node.peerId.toB58String()}`)
+
+  const conn = await node.dial(relayAddr)
+
+  console.log(`Connected to the HOP relay ${conn.remotePeer.toString()}`)
+
+  // Wait for connection and relay to be bind for the example purpose
+  node.peerStore.on('change:multiaddrs', ({ peerId }) => {
+    // Updated self multiaddrs?
+    if (peerId.equals(node.peerId)) {
+      console.log(`Advertising with a relay address of ${node.multiaddrs[0].toString()}/p2p/${node.peerId.toB58String()}`)
+    }
+  })  
+}
+
+main()