Add release note for multiple environments

Warn in the documentation that it is an experimental feature.

Change-Id: I46ea01ef9a3f6e322489e6d2a28f99d05656e0f2

doc/source/multiple-environments.rst

@@ -2,6 +2,11 @@
 Multiple Environments
 =====================
 
+.. warning::
+
+    Support for multiple Kayobe environments is considered experimental: its
+    design may change in future versions without a deprecation period.
+
 Sometimes it can be useful to support deployment of multiple environments from
 a single Kayobe configuration. Most commonly this is to support a deployment
 pipeline, such as the traditional development, test, staging and production

releasenotes/notes/multiple-environments-2961e5ba017822dd.yaml

+---
+features:
+  - |
+    Adds support for managing multiple Kayobe environments from a single
+    configuration repository. See the `documentation
+    <https://docs.openstack.org/kayobe/latest/multiple-environments.html>`__
+    for more details. Note that this feature is considered experimental: its
+    design may change in future versions without a deprecation period.