Fixed #14274 -- Added admonition about using -Wall when you run tests. Thanks...

Fixed #14274 -- Added admonition about using -Wall when you run tests. Thanks to Eric Holscher for the suggestion and draft, and to timo for contributions to the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14019 bcc190cf-cafb-0310-a4f2-bffc1f526a37