[FEAT] Add documentation on email notifications scheduled via the NotificationProtocol

[pvannierop]

Background

Since recent, Appserver van send email notifications next to push notifications. There are 2 paths to schedule notification:

1. Via notification request endpoint
2. Via NotificationProtocol

Documentation on how to trigger email notifications via route 1. was added by me in README.md (see #476).

Problem

We lack documentation on triggering email notifications via route 2 (scheduling via NotificationProtocol)

Request

Add documentation on how to trigger email notifications via the NotificationProtocol.