note on runtests.py

The tutorial didn't mention how to run tests other than in a project, so I added a couple of short paragraphs. 

I realised that https://docs.djangoproject.com/en/1.4/topics/testing/ doesn't either, and I think it should.

https://docs.djangoproject.com/en/dev/internals/contributing/writing-code/unit-tests/#quickstart does discuss it.