chore(krb5): Update readme

Author: NickLarsenNZ

krb5/README.adoc

@@ -0,0 +1,13 @@
+# krb5
+
+> [!CAUTION]
+> This image is only intended for Stackable-internal tests.
+> **It should not be considered a production-ready Kerberos KDC.**
+
+Used in:
+
+- [ad-init]
+- [hdfs-topology-provider]
+
+[ad-init]: https://github.com/stackabletech/ad-init
+[hdfs-topology-provider]: https://github.com/stackabletech/hdfs-topology-provider