Documentation issues

[tderouin]

There are some minor issues in the documentation:

    'STORAGE': {
        'BACKEND': 'django.core.files.storage.FileSystemStorage',
        # You can also use Amazon S3 or any other Django storage backends
    }

is missing a trailing comma.

python manage.py delete_thumbnails --model=app.Model --size=thumbnail_size_to_delete

--field is apparently a required parameter for this command.

Also - it would be super helpful if there were instructions on how to migrate existing models.FileField to thumbnails.fields.ImageField.