Improve READMEs and CONTRIBUTING documents

Our README and CONTRIBUTING documents may certainly be more welcome, and standardize.

Would anyone have ideas? Or source for inspiration?

My self I've spotted how Nextcloud is welcoming contributors with a dedicated landing page, and a developer manual
- https://nextcloud.com/contribute/
- https://docs.nextcloud.com/server/latest/developer_manual/getting_started/index.html

However that may request some maintenance on a regular basis

And Ruby on Rails also has a dedicated page on their website with a lot of technical details for incoming developers
- https://edgeguides.rubyonrails.org/contributing_to_ruby_on_rails.html