Extend support for the v5.8 series of releases upstream

Some folks will be unable to transition to Podman 6.0 quickly,
given it has a long list of breaking changes. Some will be
entirely unable to transition - like users of Intel-based Macs.
In the past, we have provided unofficial releases of the last
release from the previous major version for a time to help aid in
this transition. This would make the support more formal, so
people can actually refer to it, and provide a fixed end date.

Signed-off-by: Matt Heon <mheon@redhat.com>

README.md

@@ -18,6 +18,8 @@ Podman is based on libpod, a library for container lifecycle management that is
 
 Podman releases a new major or minor release 4 times a year, during the second week of February, May, August, and November. Patch releases are more frequent and may occur at any time to get bugfixes out to users. All releases are PGP signed. Public keys of members of the team approved to make releases are located [here](https://github.com/containers/release-keys/tree/main/podman).
 
+Only the most recent release receives upstream support. Exceptions are sometimes made after a major Podman release, in recognition of the fact that not all users and distributions will be able to quickly migrate to a breaking change release. After the release of Podman 6.0, the Podman maintainers have decided to extend upstream support of the v5.8 series of releases until the 2nd week of June in 2027, one year after the release of Podman 6.0. This support will only include CVE fixes and critical bugfixes.
+
 * Continuous Integration:
   * [![Build Status](https://github.com/containers/podman/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/containers/podman/actions/workflows/ci.yml?query=branch%3Amain)
   * [GoDoc: ![GoDoc](https://godoc.org/github.com/containers/podman/libpod?status.svg)](https://godoc.org/github.com/containers/podman/libpod)