Update readme with feature list

README.rst

 AlekSIS (School Information System) — App Postbuero (Mail server management)
-==================================================================================================
+============================================================================
 
 AlekSIS
 -------
@@ -9,7 +9,20 @@ This is an application for use with the `AlekSIS®`_ platform.
 Features
 --------
 
-The author of this app did not describe it yet.
+Postbuero provides integration with various mail server functionality, among which are:
+
+ * Management of supported mail domains
+ * Management of mail addresses (mailboxes) for persons
+
+   * Public registration for domains allowing it
+
+ * Management of mail addresses (aliases) for groups
+
+   * Including support for members, owners, and guardians
+
+ * `WebMilter`_ support for Postfix
+
+   * Alias resolution for persons and groups
 
 Licence
 -------
@@ -17,6 +30,7 @@ Licence
 ::
 
   Copyright © 2020 Tom Teichler <tom.teichler@teckids.org>
+  Copyright © 2022 Tom Teichler <tom.teichler@teckids.org>
 
   Licenced under the EUPL, version 1.2 or later
 
@@ -33,5 +47,6 @@ by Teckids e.V. Please refer to the `trademark policy`_ for hints on using the t
 AlekSIS®.
 
 .. _AlekSIS®: https://edugit.org/AlekSIS/official/AlekSIS
+.. _WebMilter: https://docs.bergblau.io/concepts/webmilter/
 .. _European Union Public Licence: https://eupl.eu/
 .. _trademark policy: https://aleksis.org/pages/about