Add documentation in README file

Author: NicoLaval

README.md

@@ -9,3 +9,16 @@ This API provides access to DDI metadata resources.
 ## OpenAPI Specification
 
 The API specification is available in the `ddi-rest.yaml` file in OpenAPI 3.1.1 format.
+
+## Documentation & Links
+
+### Online Documentation
+
+- **Swagger UI**: [View on GitHub Pages](https://nicolaval.github.io/DDI-API/)
+
+### Local Development
+
+To view and test the API specification locally:
+
+- **Swagger Editor**: Open [https://editor.swagger.io/](https://editor.swagger.io/) and paste the contents of `ddi-rest.yaml` or load it directly from the repository
+- **Local Swagger UI**: Run `yarn build:swagger` and `yarn preview:swagger` to start a local Swagger UI server