Add prelude for Train release

Change-Id: Ifd96a8127b9f2e398b8e853fb1de08eaf22eb696

releasenotes/notes/train-prelude-0b8a0c8fca3ff194.yaml

+---
+prelude: >
+  The Kolla Ansible ``9.0.0`` release is the first release in the Train cycle.
+  Highlights include support for deployment of the Masakari instance High
+  Availability service and Qinling Function as a Service. It is now possible
+  to deploy multiple Nova cells, and the control plane may be configured to
+  communicate via IPv6.