Missing contribution guidelines

I'd recommend adding proper labelling, `CONTRIBUTION.md` file, etc., to follow the typical rules for open-source repositories and give a possibility for the community to maintain the project

FYI @syurkevi

Start by reviewing the repository’s existing contribution and project-guidance files, then check how labels and contribution rules are currently handled. Define the scope for a CONTRIBUTION.md file and repository labelling; done means the proposed guidance and labels are present and usable by community contributors.

索引モデルが issue の本文から書いたものです。