Fix readme usage example

- Fixes ChainSafe#252

README.md

@@ -25,14 +25,15 @@ Install with `yarn add @chainsafe/libp2p-noise` or `npm i @chainsafe/libp2p-nois
 Example of using default noise configuration and passing it to the libp2p config:
 
 ```js
-import {Noise} from "@chainsafe/libp2p-noise"
+import {noise} from "@chainsafe/libp2p-noise"
 
 //custom noise configuration, pass it instead of `new Noise()`
-const noise = new Noise(privateKey, Buffer.alloc(x));
+//x25519 private key
+const n = noise(privateKey);
 
 const libp2p = new Libp2p({
    modules: {
-     connEncryption: [new Noise()],
+     connEncryption: [noise()],
    },
 });
 ```

test/index.spec.ts

@@ -4,7 +4,9 @@ import { duplexPair } from 'it-pair/duplex'
 import { pbStream } from 'it-pb-stream'
 import sinon from 'sinon'
 import { Noise } from '../src/noise.js'
+import { noise } from '../src/index.js'
 import { createPeerIdsFromFixtures } from './fixtures/peer.js'
+import { stablelib } from '../src/crypto/stablelib.js'
 
 function createCounterSpy () {
   return sinon.spy({
@@ -14,6 +16,12 @@ function createCounterSpy () {
 }
 
 describe('Index', () => {
+  it('should run the readme.md example', () => {
+    const staticNoiseKey = stablelib.generateX25519KeyPair().privateKey;
+    noise({staticNoiseKey});
+    noise();
+  })
+
   it('should expose class with tag and required functions', () => {
     const noise = new Noise()
     expect(noise.protocol).to.equal('/noise')