Add documentation for Carrier service

[nozzlegear]

Reminder for myself to add documentation for the new Carrier service.

[nozzlegear]

#183

[jeremysimmons]

if you try to modify a carrier that wasn't created by the api key that's calling, you'll get a 403 forbidden not a bug, just a note for documentation. since shopify can't be bothered to document the errors their api returns.