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!
Django seaches `LOCALE_PATHS` when it's finding the translated strings to display to the end-user, as well as where to store the strings which need to be translated. However, it only searches the current project or app to find those strings in the first place.

From the [`makemessages` docs](https://docs.djangoproject.com/en/1.7/ref/django-admin/#django-admin-makemessages):
```
django-admin.py makemessages
Runs over the entire source tree of the current directory and pulls out all strings marked for translation. 
```

Emphasis on the `current directory` part of that sentence.

I've also double checked the source code for `makemessages`. [This line](https://github.com/django/django/blob/master/django/core/management/commands/makemessages.py#L323) shows that it's only looking in the current directory.

I agree that it seems pretty naff to have to make the symlink, but this'll only need to be done for people using languages which don't ship with REST framework. I know that's all of them at the moment, but once the more common languages are translated then there shouldn't need to be many people doing this step.