message: docs: Describe WebSocket channel subscription
  In addition to bennyn/coinbase-pro-node#84
diff: @@ -66,16 +66,38 @@ console.log(message);
  **Subscribe to tickers**
  ```javascript
  -client.ws.on(WebSocketEvent.ON_MESSAGE_TICKER, message => {
  - console.log(`Received message of type "${message.type}".`);
  -});
  +const {CoinbasePro, WebSocketChannelName, WebSocketEvent} = require('coinbase-pro-node');
  -await client.ws.connect();
  +// 1. Setup Coinbase Pro client
  +const client = new CoinbasePro({
  + apiKey: '',
  + apiSecret: '',
  + passphrase: '',
  + useSandbox: false,
  +});
  -client.ws.subscribe({
  +// 2. Setup WebSocket channel info
  +const channel = {
  name: WebSocketChannelName.TICKER,
  product_ids: ['BTC-USD', 'ETH-EUR'],
  +};
  +
  +// 3. Listen to WebSocket channel updates
  +client.ws.on(WebSocketEvent.ON_MESSAGE_TICKER, message => {
  + // 6. Receive message from WebSocket channel
  + console.log(`Received message of type "${message.type}".`, message);
  + // 7. Unsubscribe from WebSocket channel
  + client.ws.unsubscribe(channel);
  + // 8. Disconnect & end program
  + client.ws.disconnect();
  });
  +
  +(async () => {
  + // 4. Connect to WebSocket
  + await client.ws.connect();
  + // 5. Subscribe to WebSocket channel
  + client.ws.subscribe(channel);
  +})();
  ```
  ## Resources
  