explain that you need to use sorl's ImageField for AdminImageMixin

If you are just looking through the docs, and end up at http://sorl-thumbnail.readthedocs.org/en/latest/examples.html#admin-examples, you'll never see the section above explaining the ImageField for models, and I and others on StackOverflow have fun into this. Here's a simple change that makes it clear.
Merge pull request #152 from mrooney/patch-1

explain that you need to use sorl's ImageField for AdminImageMixin