Add a warning for new mitaka releases of oslo.messaging

Neutron (and possibly others) were/are using a private symbol of
oslo.messaging  several reviews have been put in place to mitigate this
including a requirements change that bans
!=2.6.0,!=2.6.1,!=2.7.0,!=2.8.0,!=2.8.1,!=2.9.0 from liberty

If a new release is tagged in the range >2.6.0 <3.0.0 it will NOT be
banned and may introduce a regression.

A release in that range is doable but we need to be careful.  Add a
warning to that we don't forget and do that thing.

Depends-On: I5d15366a212c6ed21f33bfe011876fc2bb56997c
Change-Id: Ia7cdbbed5e529864e751add6af05e5fc0be963e1

deliverables/mitaka/oslo.messaging.yaml

@@ -1,4 +1,8 @@
 ---
+# NOTE(tonyb) Be careful tagging new releases < 3.0.0 due to:
+#   https://bugs.launchpad.net/neutron/+bug/1513630
+# for more information see:
+#   https://review.openstack.org/247199
 launchpad: oslo.messaging
 releases:
   - version: 2.6.0