Add release notes for 2.0.0.0-rc1

Signed-off-by: Mohammad Qureshi <47198598+qreshi@users.noreply.github.com>

release-notes/opensearch-notifications.release-notes-2.0.0.0-rc1.md

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+## Version 2.0.0.0-rc1 2022-04-25
+
+Compatible with OpenSearch 2.0.0-rc1
+
+### Features
+This is the first official release of OpenSearch Notifications.
+
+Notifications consists of `notifications-core` and `notifications` backend plugins for OpenSearch 
+and a `notificationsDashboards` frontend plugin for OpenSearch Dashboards.
+
+With Notifications you will be able to manage your notification channel configurations in a centralized location
+and send messages to these channels.
+
+Adds backend REST APIs for CRUD operations on channels and internal transport
+APIs for integrating/communicating with other plugins.