add to readme

Author: sklarsa

README.md

@@ -19,6 +19,7 @@ To use the backend, simply install the package (using pip), set the `EMAIL_BACKE
 
 1. To toggle sandbox mode (when django is running in DEBUG mode), set `SENDGRID_SANDBOX_MODE_IN_DEBUG = True/False`.
     1. To err on the side of caution, this defaults to True, so emails sent in DEBUG mode will not be delivered, unless this setting is explicitly set to False.
+1. To enable sandbox mode unconditionally (regardless of DEBUG), set `SENDGRID_SANDBOX_MODE = True`. This is useful for non-production environments like QA or staging where DEBUG may be False but you still don't want to send real emails.
 1. `SENDGRID_ECHO_TO_STDOUT` will echo to stdout or any other file-like
     object that is passed to the backend via the `stream` kwarg.
 1. `SENDGRID_TRACK_EMAIL_OPENS` - defaults to true and tracks email open events via the Sendgrid service. These events are logged in the Statistics UI, Email Activity interface, and are reported by the Event Webhook.