feat: Add OpenSearch to the README file (#541)

Author: siegfriedweber

template/.readme/partials/borrowed/footer.md.j2.j2

@@ -36,6 +36,7 @@ These are the operators that are currently part of the Stackable Data Platform:
 * [Stackable Operator for Apache Hive](https://github.com/stackabletech/hive-operator)
 * [Stackable Operator for Apache Kafka](https://github.com/stackabletech/kafka-operator)
 * [Stackable Operator for Apache NiFi](https://github.com/stackabletech/nifi-operator)
+* [Stackable Operator for OpenSearch](https://github.com/stackabletech/opensearch-operator)
 * [Stackable Operator for Apache Spark](https://github.com/stackabletech/spark-k8s-operator)
 * [Stackable Operator for Apache Superset](https://github.com/stackabletech/superset-operator)
 * [Stackable Operator for Trino](https://github.com/stackabletech/trino-operator)