Add missing CONTRIBUTING.md with contributing guidelines

[IvoPereira]

Description

The current README.md points to CONTRIBUTING.md that hasn't been yet created.

Expected result

Such file should be created and point to clear instructions on what the best practices are for contributing to this repository.

Possible inspirations:
https://github.com/Redocly/redoc/blob/master/.github/CONTRIBUTING.md

[Alf0nso]

Readme.md completed properly, at least first draft
closerino

[IvoPereira]

Readme.md completed properly, at least first draft
closerino

However a CONTRIBUTION.md file should still be added with more in-depth instructions on how to contribute, including useful tips like "writing good commit messages".