Create CONTRIBUTING.md

[jabalde]

As a developer
I want to share with everybody how to contribute with openapi-nodejs-cli
to ensure people knows how to work in openapi-nodejs-cli

Technical information
The CONTRIBUTING.md file is a text file which project managers include in free and open-source software packages for the purpose of describing how others may contribute to the project.

Acceptance criteria

• The CONTRIBUTING.md is deployed

[jabalde]

The contributing.md has been included in #7