Add a CONTRIBUTION guide

We're missing a CONTRIBUTING.md file explaning how to pick a task/what needs to be done and how to contribute to the project

Start by reviewing the repository's existing project information and open issues to understand how tasks are selected and what work needs attention. Create CONTRIBUTING.md with guidance on choosing a task and contributing; done means the guide clearly explains both processes and is present at the repository root.

Written by the indexing model from the issue text.