readme: add archive notice

Signed-off-by: Robin Hahling <[email protected]>

README.md

@@ -1,3 +1,13 @@
+> [!IMPORTANT]
+> This repository is archived and unmaintained.
+>
+> Hubble OTEL was an experimental project to integrate Hubble with
+> OpenTelemetry.
+>
+> If you are interested in a feature related to Hubble and OpenTelemetry that is
+> not yet implemented, we encourage you to open an issue on the
+> [cilium issue tracker](https://github.com/cilium/cilium/issues/new/choose).
+
 # Hubble adaptor for OpenTelemetry
 
 - User Guides