Document how to implement test mode

Given that implementers of Omnipay are looking to have one piece of code work with multiple gateways testMode provides some challenges. 

I'm not sure the best way to deal with this. I would argue that Authorize.net should implement setTestMode to mean 'use sandbox account' since that is the normal meaning for other gateways, but that would not be backward compatible. 

At minimum I thought I could document how people should implement this to achieve the desired result

Author: eileenmcnaughton

README.md

@@ -437,6 +437,25 @@ try {
 }
 ```
 
+## Test mode and developer mode
+  Most gateways allow you to set up a sandbox or developer account which uses a different url
+  and credentials. Some also allow you to do test transactions against the live site, which does
+  not result in a live transaction.
+  
+  Gateways that implement only the developer account (most of them) call it testMode. Authorize.net,
+  however, implements both and refers to this mode as developerMode.  
+  
+  When implementing with multiple gateways you should use a construct along the lines of the following:
+  
+  if ($is_developer_mode) 
+  {
+    if (method_exists($gateway, 'setDeveloperMode')) {
+        $gateway->setDeveloperMode(TRUE);
+    } else {
+      $gateway->setTestMode(TRUE);
+    } 
+  }
+
 ## Token Billing
 
 Token billing allows you to store a credit card with your gateway, and charge it at a later date.