Add some notices on handling release notes

During the recent work on release notes I got some useful lessons on reno.
It is worth documented. (Note that they will be added to the reno docs
but it is nice to have it in horizon doc too)

Also adds a link to the release note section in the contributor guide
to the release note page so that developers can find information on
the release notes more easily.

Change-Id: I0c5bd5f5e70d6c1be4eb8f8e895312205177774e

doc/source/contributor/contributing.rst

@@ -613,6 +613,16 @@ If the following applies to the patch, a release note is required:
    * It is not recommended that individual release notes use **prelude**
      section as it is for release highlights.
 
+.. 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.
+
 Horizon uses `reno <https://docs.openstack.org/reno/latest/user/usage.html>`_ to
 generate release notes. Please read the docs for details. In summary, use
 

releasenotes/source/index.rst

@@ -17,3 +17,8 @@
    grizzly
    folsom
    essex
+
+.. toctree::
+   :maxdepth: 1
+
+   Release Notes Howto <https://docs.openstack.org/horizon/latest/contributor/contributing.html#release-notes>