Document how to translate DRF error messages

Starts work on https://github.com/tomchristie/django-rest-framework/issues/2063 by describing how to translate the existing DRF error messages.

This guide follows the steps outlined in http://stackoverflow.com/a/20331816/691427 for translating 3rd party apps in Django. I've tested locally and it works for me.

I'm opening this PR for feedback, so I'd appreciate any and all comments!