Added exceptions reference page to the API reference index.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13187 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/ref/index.txt

@@ -11,6 +11,7 @@ API Reference
    contrib/index
    databases
    django-admin
+   exceptions
    files/index
    forms/index
    generic-views