docs: add link to flask-redmail

docs/extensions/flask.rst

@@ -0,0 +1,15 @@
+.. _ext-flask:
+
+Flask Extension
+===============
+
+For Flask web applications, you may use the Flask extension: 
+**Flask-Redmail**. Even though it is trivial to use Red Mail
+in such applications, Flask-Redmail integrates with Flask's
+configurations to make it slightly easier to manage.
+
+Read more:
+
+- `flask-redmail's documentation <https://flask-redmail.readthedocs.io/>`_
+- `flask-redmail's source code <https://github.com/Miksus/flask-redmail>`_
+- `flask-redmail's releases from PyPI <https://pypi.org/project/Flask-Redmail/>`_

docs/extensions/index.rst

@@ -4,10 +4,11 @@ Extensions
 
 This section covers Red Mail's extensions. Extensions are 
 tools that helps to integrate the email sender to other 
-tasks such as logging.
+purposes such as logging or web applications.
 
 .. toctree::
    :maxdepth: 1
    :caption: Contents:
 
-   logging.rst
+   logging.rst
+   flask.rst