Add a link to the OpenAPI interactive doc in the README

[cbornet]

This is a nice trick to be able to use the Nakadi API quickly by leveraging the swagger-ui hosted by swagger.io

[cbornet]

Actually there's a small issue with this : the OAS forces an HTTPS scheme so the requests will fail for localhost:8080 since there's no SSL support on this path. It's still useful since you can build your request with all the support of swagger-ui, execute the request, copy/paste the curl command and remove the "s" in "https" but it would be even better if the http scheme were also added to the OAS so the user can choose which scheme to use.