Add release note for 2.0.0 (opensearch-project#174)

* Add release note for 2.0.0

Signed-off-by: Deep Datta <deedatta@amazon.com>

Author: deepdatta

release-notes/logstash-output-opensearch.release-notes-2.0.0.md

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+## Version 2.0.0 Release Notes
+
+Compatible with OpenSearch 1.3.4, 2.1.0
+
+### Features
+
+* Configuration parameter `service_name` to specify the service name for Signature Version 4 (SigV4) signing. ([#168](https://github.com/opensearch-project/logstash-output-opensearch/pull/168))
+* Configuration parameter `legacy_template` to support index templates. ([#169](https://github.com/opensearch-project/logstash-output-opensearch/pull/169))
+* Configuration parameter `default_server_major_version` to use when the version number can't be fetched from the host's root URL. ([#170](https://github.com/opensearch-project/logstash-output-opensearch/pull/170))
+
+### Enhancements
+
+* AWS SDK dependency updated to 3.0 to work with Logstash 8.4.0 ([#171](https://github.com/opensearch-project/logstash-output-opensearch/pull/171))
+
+