update API doc for Member Registration API

Author: fireice.topcoder

apiary.apib

@@ -635,7 +635,10 @@ Register a new user.
             "country" : "UK",
             "email" : "name@domain.com",
             "password" : "HashedPassword",
-            "regSource": "http://registration.source.domain.com"
+            "regSource": "http://registration.source.domain.com",
+            "utm_source": "a",
+            "utm_medium": "b",
+            "utm_campaign": "c"
         }
 
 ### Create a new user [POST]
@@ -6140,4 +6143,4 @@ Payments APIs
             "name":"Service Unavailable",
             "value":"503",
             "description":"Servers are up but overloaded. Try again later."
-        }
+        }