[stable-em-only] Add CVE-2023-2088 warning

The Cinder project team does not intend to backport a fix for
CVE-2023-2088 to stable/wallaby, so add a warning to the README
so that consumers are aware of the vulnerability of this branch
of the cinder code.

Change-Id: I83b5232076250553650b8b97409cbf72e90c15b9
Related-bug: #2004555

README.rst

@@ -7,6 +7,22 @@ OpenStack Cinder
 
 .. Change things from this point on
 
+.. warning::
+   The stable/wallaby branch of cinder does not contain a fix for
+   CVE-2023-2088_.  Be aware that such a fix must span cinder, os-brick,
+   nova, and, depending on your deployment configuration, glance_store
+   and ironic.  *The Cinder project team advises against using the code
+   in this branch unless a mitigation against CVE-2023-2088 is applied.*
+
+   .. _CVE-2023-2088: https://nvd.nist.gov/vuln/detail/CVE-2023-2088
+
+   References:
+
+   * https://nvd.nist.gov/vuln/detail/CVE-2023-2088
+   * https://bugs.launchpad.net/cinder/+bug/2004555
+   * https://security.openstack.org/ossa/OSSA-2023-003.html
+   * https://wiki.openstack.org/wiki/OSSN/OSSN-0092
+
 OpenStack Cinder is a storage service for an open cloud computing service.
 
 You can learn more about Cinder at: