Add release notes for 1.2.0.0 release (opensearch-project#225)

* Create opensearch-alerting.release-notes-1.2.0.0.md

Signed-off-by: Annie Lee <leeyun@amazon.com>

* Update opensearch-alerting.release-notes-1.2.0.0.md

* Update opensearch-alerting.release-notes-1.2.0.0.md

release-notes/opensearch-alerting.release-notes-1.2.0.0.md

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+## Version 1.2.0.0 2021-11-05
+
+Compatible with OpenSearch 1.2.0
+
+### Enhancements
+
+* Admin Users must be able to access all monitors #139 ([#220](https://github.com/opensearch-project/alerting/pull/220))
+* Add valid search filters. ([#194](https://github.com/opensearch-project/alerting/pull/194))
+
+### Bug Fixes
+
+* Fixed a bug that was preventing the AcknowledgeAlerts API from acknowledging more than 10 alerts at once. ([#205](https://github.com/opensearch-project/alerting/pull/205))
+* Remove user from the responses ([#201](https://github.com/opensearch-project/alerting/pull/201))
+
+### Infrastructure
+
+* Update build to use public Maven repo ([#184](https://github.com/opensearch-project/alerting/pull/184))
+* Publish notification JARs checksums. ([#196](https://github.com/opensearch-project/alerting/pull/196))
+* Updates testCompile mockito version to match OpenSearch changes ([#204](https://github.com/opensearch-project/alerting/pull/204))
+* Update maven publication to include cksums. ([#224](https://github.com/opensearch-project/alerting/pull/224))
+* Updates alerting version to 1.2 ([#192](https://github.com/opensearch-project/alerting/pull/192))
+
+### Documentation
+
+* Add release notes for 1.2.0.0 release ([#225](https://github.com/opensearch-project/alerting/pull/225))
+
+### Maintenance
+
+* Update copyright notice ([#222](https://github.com/opensearch-project/alerting/pull/222))
+