feat(usermanual): Document task from email feature

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

Author: ChristophWurst

user_manual/groupware/calendar.rst

@@ -1,3 +1,5 @@
+.. _calendar-app:
+
 ======================
 Using the Calendar app
 ======================

user_manual/groupware/mail.rst

@@ -86,6 +86,18 @@ Create an event for a certain message/thread directly via mail app
 2. Click *more actions*
 3. Click *create event*
 
+Create a task
+~~~~~~~~~~~~~
+
+.. versionadded:: 3.2
+
+Create an task for a certain message/thread directly via mail app
+1. Open action menu of an envelope
+2. Click *more actions*
+3. Click *create task*
+
+.. note:: Tasks are stored in supported calendars. If there is no compatible calendar you can create a new one with the :ref:`calendar app<calendar-app>`.
+
 Edit tags
 ~~~~~~~~~~
 1. Open action menu of an envelope