Add warning to README

This clarifies that `master` is no longer actively maintained, now that
its contents have been moved over to `rancher/fleet`.
Still, the repository must live on at least as long as it has active
release branches for Fleet backports.

README.md

@@ -1,3 +1,7 @@
+> [!WARNING]
+> This repository is no longer actively maintained, except for `release/fleet/*` branches used for Fleet backports.
+> See [rancher/fleet](https://github.com/rancher/fleet) for more recent contents and issues.
+
 gitjob
 ========