Documentation: Third-party libs

PyMS use powerful libraries such as [Flask](https://github.com/pallets/flask), [Connexion](https://github.com/zalando/connexion), [Opentracing](https://github.com/opentracing/opentracing-python) but we don't explain how to use in deep this libraries. Add more links, sources and reviews of this libraries

Moved from https://github.com/python-microservices/pyms/issues/163