Allow Mandrill specific options to be set globally in the settings file.

This is useful to set options such as tracking_domain etc per instance when using subaccounts with Mandrill.

docs/usage/sending_mail.rst

@@ -103,11 +103,21 @@ Some notes and limitations:
 Mandrill-Specific Options
 -------------------------
 
+.. setting:: MANDRILL_SETTINGS
+
 Most of the options from the Mandrill
 `messages/send API <https://mandrillapp.com/api/docs/messages.html#method=send>`_
 `message` struct can be set directly on an :class:`~django.core.mail.EmailMessage`
 (or subclass) object:
 
+Most of these options can be globally set in your project's :file:`settings.py`
+using :setting:`MANDRILL_SETTINGS`. For Example::
+
+    MANDRILL_SETTINGS = {
+        'tracking_domain': 'example.com',
+        'track_opens': True,
+    }
+
 .. These attributes are in the same order as they appear in the Mandrill API docs...
 
 .. attribute:: important