Create contribution guidelines

[mdelapenya]

As we are receiving more contributions 🚀 🎸 we'd need to define how to contribute in a more standard manner.

Things that I like when doing a review of a PR:

• has a self-explanatory title
• has a great description where the what and why is properly explained
• has tests
• is never push-forced
• updates the docs
• passes the CI checks
• passes the lint checks
• when needed, contains links to existing issues/PRs

What things do you see important to address for this project? We could create a PR template including those checks

[mdelapenya]

We submitted an initial PR with the contributing guides here: #522