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!
The approach of having to symlink to rest_framework seems very obscure - surely simply having `rest_framework` in the `INSTALLED_APPS` is enough for the `makemessages` command to detect the localized text strings?