Add WIP note to readme

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

Author: stefanprodan

README.md

@@ -1,2 +1,11 @@
 # plugins
-Plugin catalog for the Flux CLI
+
+[![license](https://img.shields.io/github/license/fluxcd/plugins.svg)](https://github.com/fluxcd/plugins/blob/main/LICENSE)
+
+This repository contains the catalog of Flux CLI plugins as specified
+in the [Flux CLI Plugin System RFC](https://github.com/fluxcd/flux2/pull/5795).
+
+> [!NOTE]
+> This repository is in early development and the plugin system is not yet available in a stable release of Flux.
+> The instructions for installing and using the plugins will be added here once the system is ready for testing.
+> In the meantime, you can follow the progress of the plugin system in the Flux CLI Plugin System RFC linked above.