Document iMIP API change for 26

Signed-off-by: Anna Larch <anna@nextcloud.com>

developer_manual/digging_deeper/groupware/calendar.rst

@@ -161,6 +161,27 @@ Calendars that only return `ICalendar` are implicitly read-only. If your app's c
 
     }
 
+Handling iMIP data 
+~~~~~~~~~~~~~~~~~~
+
+You may implement the ``IHandleIMipMessage`` interface to process iMIP data you receive in a client and want to pass on for processing to the backend. 
+
+Please be aware that there are some security considerations to take into account. You can find more infomation on these and the conditions that have to be fulfilled for iMIP data to be processed in the `RFC <https://www.rfc-editor.org/rfc/rfc6047>`_
+
+.. code-block:: php
+
+    <?php
+
+    use OCP\Calendar\IHandleIMipMessage;
+
+    class HandleIMipMessage implements IHandleIMipMessage {
+
+        public function handleIMipMessage(string $name, string $calendarData): void {
+            // Validation and write to your calendar representation
+        }
+
+    }
+
 Resources
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