Add mailman3 RPM build investigation

Signed-off-by: Michal Konečný <mkonecny@redhat.com>

docs/mailman3/index.rst

@@ -0,0 +1,33 @@
+Mailman3
+========
+
+Purpose
+-------
+
+Purpose of this investigation is to find a possible ways to migrate CentOS mailing lists to
+mailman3 from the current solution and look at them more closely. The output of the investigation
+is this document.
+
+Resources
+---------
+
+* Mailman3 upstream: https://wiki.list.org/Mailman3
+* Mailman3 Fedora dist-git: https://src.fedoraproject.org/rpms/mailman3
+
+Investigation
+-------------
+
+Here is the list of ideas/things we looked at:
+
+.. toctree::
+    :maxdepth: 1
+
+    building_rpm_centos_stream
+
+
+Conclusions
+-----------
+
+
+Proposed roadmap
+----------------