Missing CONTRIBUTING.md

[onel]

Feature request

The repo should add a CONTRIBUTING.md

Motivation

To help new contributors

Your contribution

I can help with adding one if there is a widely used template
For example: https://github.com/huggingface/datasets/blob/main/CONTRIBUTING.md

[vrdn-23]

+1 for this

[OlivierDehaene]

Fixed by #291