Add changelog

Author: siegfriedweber

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+## [Unreleased]
+
+### Added
+
+- Basic operator for OpenSearch 3.x with the following configuration options ([#10]):
+  - Cluster operations like `reconciliationPaused` and `stopped`
+  - Image selection (defaults to the official OpenSearch image for now)
+  - Overrides (CLI, config, environment variables, Pod)
+  - Affinities
+  - Graceful shutdown timeout
+  - OpenSearch node roles
+  - Resources (CPU, memory, storage)
+  - PodDisruptionBudgets
+  - Replicas
+
+[#10]: https://github.com/stackabletech/opensearch-operator/pull/10