Fix doc about test discovery

I had my tests in a file called tests.py and they weren't being discovered. 

I think py.test looks for files of the form test_*.py , and not test*.py; could you confirm that? (See last paragraph http://pytest.org/latest/goodpractises.html#python-test-discovery )

Another thing we could suggest is to set python_files=test*.py for this case.
Merge pull request #69 from ustun/patch-1

Fix doc about test discovery