message: [ReadMe] Update links to OpenAPI spec
  Update v2.0 to point to the version hosted by the OpenAPI Initiative
  Update v3.0.0 to point to the released 3.0.0 version
diff: 7. Contributing to the code
  8. OpenAPI (formerly 'Swagger') specifications:
  -- [Swagger 2.0](https://github.com/swagger-api/swagger-spec/blob/master/versions/2.0.md)
  -- [OpenAPI 2.0](https://github.com/OAI/OpenAPI-Specification/blob/3.0.0-rc0/versions/3.0.md)
  +- [OpenAPI Spec 2.0](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md) (f.k.a. Swagger 2.0)
  +- [OpenAPI Spec 3.0.0](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md)
  