Add CHANGELOG (#803)

Signed-off-by: Heemin Kim <[email protected]>

.github/workflows/changelog_verifier.yml

@@ -0,0 +1,18 @@
+name: "Changelog Verifier"
+on:
+  pull_request:
+    types: [opened, edited, review_requested, synchronize, reopened, ready_for_review, labeled, unlabeled]
+
+jobs:
+  # Enforces the update of a changelog file on every pull request
+  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

@@ -0,0 +1,11 @@
+# CHANGELOG
+Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
+
+## [Unreleased 1.x](https://github.com/opensearch-project/OpenSearch/compare/1.3.9...HEAD)
+### Features
+### Enhancements
+### Bug Fixes
+### Infrastructure
+### Documentation
+### Maintenance
+### Refactoring

CONTRIBUTING.md

@@ -8,6 +8,7 @@ OpenSearch k-NN is a community project that is built and maintained by people ju
 - [Ways to Contribute](#ways-to-contribute)
 - [Developer Certificate of Origin](#developer-certificate-of-origin)
 - [License Headers](#license-headers)
+- [Changelog](#changelog)
 - [Review Process](#review-process)
 
 
@@ -129,6 +130,42 @@ New files in your code contributions should contain the following license header
 # SPDX-License-Identifier: Apache-2.0
 ```
 
+## Changelog
+
+OpenSearch maintains version specific changelog by enforcing a change to the ongoing [CHANGELOG](CHANGELOG.md) file adhering to the [Keep A Changelog](https://keepachangelog.com/en/1.0.0/) format. The purpose of the changelog is for the contributors and maintainers to incrementally build the release notes throughout the development process to avoid a painful and error-prone process of attempting to compile the release notes at release time. On each release the "unreleased" entries of the changelog are moved to the appropriate release notes document in the `./release-notes` folder. Also, incrementally building the changelog provides a concise, human-readable list of significant features that have been added to the unreleased version under development.
+
+### Which changes require a CHANGELOG entry?
+Changelogs are intended for operators/administrators, developers integrating with libraries and APIs, and end-users interacting with OpenSearch Dashboards and/or the REST API (collectively referred to as "user"). In short, any change that a user of OpenSearch might want to be aware of should be included in the changelog. The changelog is _not_ intended to replace the git commit log that developers of OpenSearch itself rely upon. The following are some examples of changes that should be in the changelog:
+
+- A newly added feature
+- A fix for a user-facing bug
+- Dependency updates
+- Fixes for security issues
+
+The following are some examples where a changelog entry is not necessary:
+
+- Adding, modifying, or fixing tests
+- An incremental PR for a larger feature (such features should include _one_ changelog entry for the feature)
+- Documentation changes or code refactoring
+- Build-related changes
+
+Any PR that does not include a changelog entry will result in a failure of the validation workflow in GitHub. If the contributor and maintainers agree that no changelog entry is required, then the `skip-changelog` label can be applied to the PR which will result in the workflow passing.
+
+### How to add my changes to [CHANGELOG](CHANGELOG.md)?
+
+Adding in the change is two step process:
+1. Add your changes to the corresponding section within the CHANGELOG file with dummy pull request information, publish the PR
+2. Update the entry for your change in [`CHANGELOG.md`](CHANGELOG.md) and make sure that you reference the pull request there.
+
+### Where should I put my CHANGELOG entry?
+Please review the [branching strategy](https://github.com/opensearch-project/.github/blob/main/RELEASING.md#opensearch-branching) document. The changelog on the `main` branch will contain sections for the _next major_ and _next minor_ releases. Your entry should go into the section it is intended to be released in. In practice, most changes to `main` will be backported to the next minor release so most entries will likely be in that section.
+
+The following examples assume the _next major_ release on main is 3.0, then _next minor_ release is 2.5, and the _current_ release is 2.4.
+
+- **Add a new feature to release in next minor:** Add a changelog entry to `[Unreleased 2.x]` on main, then backport to 2.x (including the changelog entry).
+- **Introduce a breaking API change to release in next major:** Add a changelog entry to `[Unreleased 3.0]` on main, do not backport.
+- **Upgrade a dependency to fix a CVE:** Add a changelog entry to `[Unreleased 2.x]` on main, then backport to 2.x (including the changelog entry), then backport to 2.4 and ensure the changelog entry is added to `[Unreleased 2.4.1]`.
+
 ## Review Process
 
 We deeply appreciate everyone who takes the time to make a contribution.  We will review all contributions as quickly as possible.  As a reminder, [opening an issue](https://github.com/opensearch-project/k-NN/issues/new/choose) discussing your change before you make it is the best way to smooth the PR process.  This will prevent a rejection because someone else is already working on the problem, or because the solution is incompatible with the architectural direction.