Add the documentation for the Email Notification plugin

We have added documentation for the Email notification plugin to the Configuring Notification
Plugins section. Previously, the related information could be found in the Configuring an SMTP
Server section within Configuring Django. We have moved the Promgen-related configuration to
the newly created section and retained only the Django-related configuration in the Configuring
Django section.

Author: hoangpn

docs/conf/django.rst

@@ -9,17 +9,16 @@ All available django settings (not every setting may apply to promgen)
 are listed in the `django
 reference <https://docs.djangoproject.com/en/1.11/ref/settings/>`__.
 
+.. _smtp_config:
 Configuring an SMTP Server
 --------------------------
 
 An SMTP server for sending outgoing mail can be configured this way:
 
 .. code-block:: yaml
 
-    promgen.notification.email:
-      sender: promgen@example.com
-
     django:
+      DEFAULT_FROM_EMAIL: promgen@example.com
       EMAIL_HOST: mail.example.com
       EMAIL_PORT: 587
       EMAIL_HOST_USER: user@example.com

docs/conf/notification.rst

@@ -3,6 +3,20 @@ Configuring Notification Plugins
 
 Different settings for the Notification Plugins can be defined in the ``promgen.yml`` file.
 
+Email
+---------------------
+
+An SMTP server for sending outgoing mail can be configured by adding Django email settings
+(see :ref:`smtp_config`).
+The **email's sender address** either can be set using the ``DEFAULT_FROM_EMAIL`` Django setting
+or by setting the ``sender`` configuration as shown below. If both are set, the ``sender``
+configuration will take precedence.
+
+.. code-block:: yaml
+
+    promgen.notification.email:
+      sender: promgen@example.com
+
 LINE Notify
 ---------------------