add usage to README (#52)

README.md

@@ -190,7 +190,23 @@ TODO
 
 ## Usage
 
-TODO
+Once your Django project is configured to use `email_relay.backend.RelayDatabaseEmailBackend` as it's `EMAIL_BACKEND`, sending email is as simple as using Django's built-in ways of sending email, such as the `send_mail` method:
+
+```python
+from django.core.mail import send_mail
+
+send_mail(
+    "Subject here",
+    "Here is the message.",
+    "from@example.com",
+    ["to@example.com"],
+    fail_silently=False,
+)
+```
+
+Any emails sent this way, or one of the other ways Django provides, will be stored in the database queue and sent by the relay service.
+
+See the Django documentation on [sending email](https://docs.djangoproject.com/en/dev/topics/email/) for more information.
 
 ## Configuration