Add custom guzzle client example to docs (#169)

README.md

@@ -573,6 +573,9 @@ print_r($response->data);
 
 ## Troubleshooting
 
+
+### `unable to get local issuer certificate`
+
 Some users have issues making requests due to the following error:
 
 ```
@@ -592,6 +595,17 @@ curl.cainfo = "/etc/pki/tls/cacert.pem"
 curl.cainfo = "C:\php\extras\ssl\cacert.pem"
 ```
 
+### Pass custom Guzzle client
+
+We allow use of any HTTPlug adapter, so you can create a client with alternative configuration if you need it, for example to take account of a local proxy, or deal with something else specific to your setup.
+
+Here's an example that reduces the default timeout to 5 seconds to avoid long delays if you have no route to our servers:
+
+```
+$adapter_client = new Http\Adapter\Guzzle6\Client(new GuzzleHttp\Client(['timeout' => 5]));
+$nexmo_client = new Nexmo\Client(new Nexmo\Client\Credentials\Basic($api_key, $api_secret), [], $adapter_client);
+```
+
 API Coverage
 ------------