docs: Add section on email marketing configuration in CRM settings

Author: Vadym-Kharchenko

docs/site/django_crm_settings.md

@@ -67,6 +67,12 @@ Specify details for connecting to an email account through which CRM will be abl
 Add the addresses of CRM administrators to the list, so they can receive error logs.  
 `ADMINS = [("<Admin1 name>", "<admin1_box@example.com>"), (...)]`
 
+### CRM email marketing
+
+This is the **mailing CRM**, so email campaigns are allowed by default.  
+If you do not intend to use them, set the `MAILING` parameter to `False` (recommended).  
+Learn more about this [CRM and email marketing](https://djangocrm.github.io/info/features/massmail-app-features){target="_blank"}.
+
 ## CRM and database testing
 
 Run the built-in tests: