Update README.md

Sorl Thumnail currently requires THUMBNAIL_DEBUG = False

Hi as of the time of writing, the version of sorl-thumbnail installed, 11.12.1b, has a requirement that THUMBNAIL_DEBUG = False otherwise it fails when there are errors, and it's quite hard to debug on a remote server.

I have just added a document patch so others don't have the issue.

This pull request to sorl-thumbnail gave me the clue and enabled me to work round the problem:
https://github.com/mariocesar/sorl-thumbnail/pull/261