mark the project as no longer maintained

When this is merged, the project will be archived and a similar note will be added to the [kargo-render-action](https://github.com/akuity/kargo-render-action) README, which will also be archived.

Signed-off-by: Kent Rancourt <kent.rancourt@gmail.com>

Author: krancour

README.md

@@ -1,3 +1,19 @@
+> ⚠️ Kargo Render is no longer maintained.
+>
+> Kargo Render was extracted from [Kargo](https://github.com/akuity/kargo)
+> when it was a younger project in order to make parts of it having to do with
+> the well-proven rendered branches pattern more immediately accessible, without
+> requiring users to buy into a larger body of more experimental concepts. Since
+> that time, Kargo has matured significantly, and since prior to its GA release,
+> its robust support for the rendered branches pattern no longer relies on Kargo
+> Render. At this time, Kargo's maintainers are confident that anything that
+> could be done with Kargo Render can now be done better by Kargo itself.
+>
+> If you are among those who found Kargo Render to be useful, we invite you to
+> now consider using [Kargo](https://github.com/akuity/kargo) instead.
+
+---
+
 # Kargo Render -- Rendered Branches Utility
 
 ![CI](https://github.com/akuity/kargo-render/actions/workflows/ci.yaml/badge.svg)