Add developer guide/integration test section (opensearch-project#4330)

* Add developer guide/integration test section

Aligned with Kafka example
https://github.com/opensearch-project/data-prepper/blob/51ee0df595aaad1b921c888394bf3e110ffc74e9/data-prepper-plugins/kafka-plugins/README.md#developer-guide

Signed-off-by: Jürgen Walter <juergen.walter@sap.com>

* Use latest OpenSearch for testing

Version 2.12.0 onwards requires to use non-default admin password
https://opensearch.org/blog/replacing-default-admin-credentials/

Signed-off-by: Jürgen Walter <juergen.walter@sap.com>

---------

Signed-off-by: Jürgen Walter <juergen.walter@sap.com>

data-prepper-plugins/opensearch/README.md

@@ -596,3 +596,20 @@ The default behavior is to process all indices.
 #### <a name="index_configuration">Index Configuration</a>
 
 * `index_name_regex`: A regex pattern to represent the index names for filtering
+
+## Developer guide
+
+### Integration tests
+
+#### Run OpenSearch
+
+Start an instance of OpenSearch that listens to opens port 9200 with default user admin:admin.
+```
+docker run -p 9200:9200 -e "discovery.type=single-node" -e "OPENSEARCH_INITIAL_ADMIN_PASSWORD=yourStrongPassword123!" opensearchproject/opensearch:latest
+```
+
+#### Run tests
+
+```
+./gradlew data-prepper-plugins:opensearch:integrationTest -Dtests.opensearch.host=localhost:9200 -Dtests.opensearch.user=admin -Dtests.opensearch.password=yourStrongPassword123!
+```