Add instructions on how to add test-only models

[bmispelon]

I was trying to use some models only for my tests and it took me a while to figure it out.

In the end, I managed to do it by adding 'tests' to the INSTALLED_APPS in runtests.py. I'm not sure if that's the best way to go about it but it does seem to work in my case.

[jonathan-s]

Do you think you could explain what you mean with models only for tests? Do you mean an example app that uses the models in the real django app?