WIP: Integrating django-secure

For background info, please see the [django-developers thread](https://groups.google.com/d/topic/django-developers/t8ybImtdnpM/discussion).
The warnings in the docs look good, and I agree with keeping messages short and expanding in the docs; but I'm concerned that in this warning we are telling a user to set a setting that could cause serious unfixable problems on their production site, without so much as telling them "hey, you really need to read the docs before you set this!"

Maybe a much shorter parenthetical addition, like "(be sure to read the documentation first; enabling HSTS carelessly can cause serious problems.)"?