Add warning about support and what to do

Author: stevepiercy

plone/content.md

@@ -8,6 +8,12 @@
 
 The official Docker images for Plone 5.x and Plone 4.x are available here.
 
+**IMPORTANT**
+
+-   Plone 4.x is no longer supported.
+-   Plone 5.x is out of maintenance support, and will exit security support on 2024-10-31.
+-   It is strongly recommended to migrate to the latest available Plone version. See https://plone.org/download/release-schedule for details.
+
 Beginning with Plone 6.x, the frontend and backend run in separate Docker images.
 
 -   [Plone Backend](https://github.com/plone/plone-backend)