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!
Does setting `LOCALE_PATHS` for translation files of non-local apps would make sense instead?

https://docs.djangoproject.com/en/1.7/ref/settings/#locale-paths