Exclude relnote from past releases from Pike relnotes

reno includes release notes touched in a target release
even if they are added from past releases.
The recent release note cleanup I made as part of doc8 adoption
and some other cleanup touched release notes from past releases.
As a result, we see several number of release notes from past
release in the Pike release notes unexpectedly.

Also adds a warning note to the release notes howto page

Change-Id: I127f96896488e9992d9309d76055b4883f6a62f3
Closes-Bug: #1708560

releasenotes/source/README.rst

@@ -6,5 +6,21 @@ Release notes are a new feature for documenting new features in
 OpenStack projects. Background on the process, tooling, and
 methodology is documented in a `mailing list post by Doug Hellmann <http://lists.openstack.org/pipermail/openstack-dev/2015-November/078301.html>`_.
 
+Writing release notes
+---------------------
+
 For information on how to create release notes, please consult the
-`Release Notes documentation <https://docs.openstack.org/reno/latest/>`_.
+`reno documentation <https://docs.openstack.org/reno/latest/user/usage.html>`__.
+
+Maintaining release notes
+-------------------------
+
+.. warning::
+
+   Avoid modifying an existing release note file even though it is related
+   to your change. If you modify a release note file of a past release,
+   the whole content will be shown in a latest release. The only allowed
+   case is to update a release note in a same release.
+
+   If you need to update a release note of a past release,
+   edit a corresponding release note file in a stable branch directly.

releasenotes/source/unreleased.rst

@@ -1,5 +1,7 @@
-=============================
- Current Series Release Notes
-=============================
+============================
+Current Series Release Notes
+============================
 
 .. release-notes::
+   :ignore-notes:
+     vlan-aware-vms-aka-trunk-3341cc75ba1bf5b4.yaml