title: Asking to do a POST with RestKit using CoreData Tutorial/Example
body: <p>I want to do a POST of an CoreData entity called 'Contacts' but in the JSON file who I will send I need to include others params like the 'UserId' of the user who has this contacts. 
  I think that RestKit with CoreData only sends the full entity of 'Contacts' without any other params.
  Anyone knows how to do it?.
  Thanks</p>