add post about mailing list migration

posts/improved-interface-for-mailing-list.rst

+.. title: Improved web interface for the mailing list
+.. slug: improved-interface-for-mailing-list
+.. date: 2020-05-28 20:00:00 UTC
+.. category: 
+.. type: text
+
+The Kwant mailing list has been migrated to a new server that provides a much-improved web interface.
+
+.. TEASER_END
+
+In May 2020, thanks to the friendly people at python.org,
+kwant-discuss was migrated to a server running Mailman 3.
+The list remains a mailing list (existing subscriptions continue),
+but now also offers a modern web interface that is similar to a web forum.
+The complete archives were migrated and are now searchable.
+Please see the `“community” section </community>`_ of this site for usage information.
+The `old address of the list <kwant-discuss@kwant-project.org>`_ remains usable: any messages are simply redirected to the new one.