Update README.md (#336)

Provide announcement / redirect to Apache Iceberg project which contains the current, maintained code.

Author: liko9

README.md

@@ -1,3 +1,6 @@
+# THIS REPOSITORY IS NOT MAINTAINED AS THE CODE HAS BEEN DONATED TO THE UPSTREAM APACHE ICEBERG PROJECT
+Please visit the [Documentation](https://iceberg.apache.org/docs/nightly/kafka-connect/) or find the current maintained source code [here](https://github.com/apache/iceberg/tree/main/kafka-connect)
+
 # Apache Iceberg Sink Connector
 The Apache Iceberg Sink Connector for Kafka Connect is a sink connector for writing data from Kafka into Iceberg tables.