docs: add OpenAPI validation steps and PR checklist

README.md

@@ -16,6 +16,30 @@ The repository currently contains specifications for:
 > [!IMPORTANT]
 > NOTE: Usage of the API are subject to additional Terms and Conditions.
 
+## Contributing / Making changes
+
+This repository contains OpenAPI YAML specifications. A good first contribution is often improving documentation, fixing small typos, or making a small, well-scoped spec improvement (with validation).
+
+For contribution guidelines and CLA requirements, see [CONTRIBUTING.md](CONTRIBUTING.md).
+
+> Note: This GitHub repository is generated from an internal Meta repository. PRs are reviewed on GitHub, then imported internally by Meta maintainers. You may not see a traditional “merge commit”, but accepted changes will appear once synced. (Details in `CONTRIBUTING.md`.)
+
+## Validating the OpenAPI spec locally
+
+Before opening a PR that changes any `.yaml` spec, it’s recommended to run a validator/linter locally.
+
+### Option 1: Redocly CLI (recommended)
+```bash
+npm install --global @redocly/cli
+redocly lint business-messaging-api_v23.0.yaml
+```
+
+### Option 2: swagger-cli
+```bash
+npm install --global swagger-cli
+swagger-cli validate business-messaging-api_v23.0.yaml
+```
+
 
 ## License