Include instructions on APP_DIRS in Prerequisites docs

[timohermans]

So I was having TemplateDoesNotExist errors when trying to set up everything.

It took me some time to realize that for this package to work, you need to have APP_DIRS: True for debug_toolbar to include its templates in the projects.

Perhaps it's a good idea to include this in the documentation?

[tim-schilling]

Yeah as a temporary solution that'd be good to put in the troubleshooting space. However, ideally the toolbar would work without it and if that's not possible, it'd fail gracefully and log a warning.

[matthiask]

The django.contrib.sitemaps app also requires "APP_DIRS": True; we could have a look there how "APP_DIRS": False is handled. I couldn't find a check or anything (I didn't look long) so maybe it's enough to document the expectation that templates can be loaded from app dirs.

(I haven't used "APP_DIRS": False or anything equivalent ever since beginning to use Django before 1.0, maybe this is exotic enough that we could simply ignore it. At least if it proves too much work to add a system check or something.)

[tim-schilling]

I don't see any checks or error handling in sitemaps. And I think attempting to hijack the settings to make it work is a poor idea.