Docs update to go with PR on cardReference (thephpleague#630)

thephpleague/omnipay-common#243

Author: eileenmcnaughton

README.md

@@ -470,6 +470,8 @@ $response->getMessage(); // a message generated by the payment gateway
 ```
 
 In addition, most gateways will override the response object, and provide access to any extra fields returned by the gateway.
+If the payment authorization is re-usable the gateway will implement ```$response->getCardReference();```. This
+method is always available (but may return NULL) from 3.1.1
 
 ### Redirect Response