Fixed #13662 -- Added an entry in the README to direct people to the...

Fixed #13662 -- Added an entry in the README to direct people to the instructions for running the test suite, and cleaned up that section of the docs. Thanks to mir for the report, and to cogat and gg for the draft text.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15342 bcc190cf-cafb-0310-a4f2-bffc1f526a37