Add github.com/pb33f/libopenapi (#1004)

* Add github.com/pb33f/libopenapi

it looks like a reasonable alternative

* Update README.md

---------

Co-authored-by: Pierre Fenoll <pierrefenoll@gmail.com>

README.md

@@ -31,6 +31,7 @@ Here's some projects that depend on _kin-openapi_:
   * (Feel free to add your project by [creating an issue](https://github.com/getkin/kin-openapi/issues/new) or a pull request)
 
 ## Alternatives
+* [libopenapi](https://github.com/pb33f/libopenapi) a fully featured, high performance OpenAPI 3.1, 3.0 and Swagger parser, library, validator and toolkit
 * [go-swagger](https://github.com/go-swagger/go-swagger) stated [*OpenAPIv3 won't be supported*](https://github.com/go-swagger/go-swagger/issues/1122#issuecomment-575968499)
 * [swaggo](https://github.com/swaggo/swag) has an [open issue on OpenAPIv3](https://github.com/swaggo/swag/issues/386)
 * [go-openapi](https://github.com/go-openapi)'s [spec3](https://github.com/go-openapi/spec3)