Add train-prelude release note

This is essentially a duplicate of the cycle highlights provided
with the release, plus a warning about needing to upgrade to extracted
placement in stein before the train upgrade.

Few links are provided because the rest of the release notes are very
short and will be presented adjacent to the prelude, and they have
links.

Change-Id: I872271688fbee69ff35bf44ef82fad9ab34f5229

releasenotes/notes/train-prelude-06739452ba2f66d9.yaml

@@ -0,0 +1,23 @@
+---
+prelude: |
+    The 2.0.0 release of placement is the first release where placement is
+    available solely from its own project and must be installed separately from
+    nova. If the extracted placement is not already in use, prior to upgrading
+    to Train, the Stein version of placement must be installed. See
+    `Upgrading from Nova to Placement`_ for details.
+
+    2.0.0 adds a suite of features which, combined, enable targeting candidate
+    providers that have complex trees modeling NUMA layouts, multiple devices,
+    and networks where affinity between and grouping among the members of the
+    tree are required. These features will help to enable NFV and other high
+    performance workloads in the cloud.
+
+    Also added is support for forbidden aggregates which allows groups of
+    resource providers to only be used for specific purposes, such as reserving
+    a group of compute nodes for licensed workloads.
+
+    Extensive benchmarking and profiling have led to massive performance
+    enhancements, especially in environments with large numbers of resource
+    providers and high concurrency.
+
+    .. _Upgrading from Nova to Placement: https://docs.openstack.org/placement/latest/upgrade/to-stein.html