add note about JSON output body type

README.md

@@ -234,7 +234,7 @@ The callback argument gets 3 arguments:
 
 1. An `error` when applicable (usually from [`http.ClientRequest`](http://nodejs.org/api/http.html#http_class_http_clientrequest) object)
 2. An [`http.IncomingMessage`](http://nodejs.org/api/http.html#http_http_incomingmessage) object
-3. The third is the `response` body (`String` or `Buffer`)
+3. The third is the `response` body (`String` or `Buffer`, or JSON object if the `json` option is supplied)
 
 ## Convenience methods