Added CHANGELOG. (#309)

Signed-off-by: dblock <[email protected]>

.github/workflows/changelog.yml

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+name: Verify Changelog
+on:
+  pull_request:
+    types: [opened, edited, review_requested, synchronize, reopened, ready_for_review, labeled, unlabeled]
+
+jobs:
+  verify-changelog:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          ref: ${{ github.event.pull_request.head.sha }}
+
+      - uses: dangoslen/changelog-enforcer@v3
+        with:
+          skipLabels: "autocut, skip-changelog"

CHANGELOG.md

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+# CHANGELOG
+
+Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
+
+## [Unreleased]
+
+### Added
+
+- Added CHANGELOG ([#309](https://github.com/opensearch-project/opensearch-api-specification/pull/309))
+- Added a spec test framework ([#299](https://github.com/opensearch-project/opensearch-api-specification/pull/299))
+- Added workflow to determine API changes ([#297](https://github.com/opensearch-project/opensearch-api-specification/pull/297))
+- Added link checking ([#269](https://github.com/opensearch-project/opensearch-api-specification/pull/269))
+- Added API coverage ([#210](https://github.com/opensearch-project/opensearch-api-specification/pull/210))
+
+### Changed
+
+- Replaced Smithy with a native OpenAPI spec ([#189](https://github.com/opensearch-project/opensearch-api-specification/issues/189))
+
+### Deprecated
+
+### Removed
+
+### Fixed
+
+- Fixed GitHub pages ([#215](https://github.com/opensearch-project/opensearch-api-specification/pull/215))
+
+### Security
+
+[Unreleased]: https://github.com/opensearch-project/opensearch-api-specification/commits/main/