Fixed #7513 -- Added a note on the dependencies that need to be installed when...

Fixed #7513 -- Added a note on the dependencies that need to be installed when running the Django system tests. Thanks to Peter Melvyn for the original report, and Ramiro Morales for the original draft.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7718 bcc190cf-cafb-0310-a4f2-bffc1f526a37