Add guzzle config to readme (#1625)

Author: jdpedrie

README.md

@@ -313,6 +313,27 @@ Now all calls made by this library will appear in the Charles UI.
 
 One additional step is required in Charles to view SSL requests. Go to **Charles > Proxy > SSL Proxying Settings** and add the domain you'd like captured. In the case of the Google APIs, this is usually `*.googleapis.com`.
 
+### Controlling HTTP Client Configuration Directly
+
+Google API Client uses [Guzzle](http://docs.guzzlephp.org) as its default HTTP client. That means that you can control your HTTP requests in the same manner you would for any application using Guzzle.
+
+Let's say, for instance, we wished to apply a referrer to each request.
+
+```php
+use GuzzleHttp\Client;
+
+$httpClient = new Client([
+    'headers' => [
+        'referer' => 'mysite.com'
+    ]
+]);
+
+$client = new Google_Client();
+$client->setHttpClient($httpClient);
+```
+
+Other Guzzle features such as [Handlers and Middleware](http://docs.guzzlephp.org/en/stable/handlers-and-middleware.html) offer even more control.
+
 ### Service Specific Examples ###
 
 YouTube: https://github.com/youtube/api-samples/tree/master/php