Document the fact that you shouldn't use StaticLiveServerTestCase with WhiteNoise

[evansd]

See #86 and #204

[gregsadetsky]

@evansd Thank you for this issue! It was indexed by Google, and so I was able to troubleshoot the error I was having on Heroku CI (404 files, as noted in #204).

It might also be worth documenting that, for Heroku CI, (just as for other CI services) a test-setup key needs to be added in app.json to run collectstatic ..?

Thank you for WhiteNoise, it's really great! Cheers