Add Python frameworks, Dredd, and relevant books and articles

[abunuwas]

Thanks for this awesome list of REST resources!

This PR Adds:

• An article about documentation-driven development for REST APIs
• A book about designing REST APIs using OpenAPI
• A book about designing microservice APIs, indicating the relevant chapters for REST
• 3 popular Python frameworks for REST APIs: Connexion, flask-smorest, and flask-restx
• API testing tool Dredd, which integrates seamlessly with REST APIs and helps to make sure API implementations comply with the specification

[fzaninotto]

Hi, and thanks for your contribution.

We carefully screen all proposed links to offer a list with only th most relevant or popular resources on the subject. We don't look for completeness.

As such, your PR contains links that we'd link to keep, and others that we aren't interested in.

• Links to book probably need a section on their own.
• links to flask forks shouldn't figure in the list as they aren't popular enough