Fixed #5430 -- Improved docs for running Django's own unit tests in...

Fixed #5430 -- Improved docs for running Django's own unit tests in docs/contributing.txt. Thanks, akaihola

git-svn-id: http://code.djangoproject.com/svn/django/trunk@6295 bcc190cf-cafb-0310-a4f2-bffc1f526a37