Add CONTRIBUTING.md for Contributor Guidelines

Description

Create a CONTRIBUTING.md file that explains how developers should contribute to the project.
Review established open-source repositories for examples of common contribution guidelines and adapt the structure to fit this project.

Some repos to take inspiration:
https://github.com/github/docs/blob/main/.github/CONTRIBUTING.md
https://github.com/django/django/blob/main/CONTRIBUTING.rst

Review the linked CONTRIBUTING.md and CONTRIBUTING.rst examples first, then create CONTRIBUTING.md at the repository root. Done means the project has contributor guidelines adapted to its needs and the document is clear enough for developers to follow.

Written by the indexing model from the issue text.