Add a CONTRIBUTING.md file for offline contribution access

[guidettj]

Hi,
I noticed that the current contribution guidelines are accessible via a link in the README, which redirects to the official documentation.

However, for contributors working locally without internet access, it could be helpful to have these guidelines directly available in the repository as a CONTRIBUTING.md file. Additionally, many open source projects include a CONTRIBUTING.md file at the root of the project to centralize and standardize contribution guidelines.

I would be happy to submit a pull request to add this file, containing essential information extracted from the online documentation. This would ensure that contributors can access the guidelines even when working offline.

Do you think this would be a valuable addition to the repository?