Introduce a CONTRIBUTING.md to guide potential contributors

[scroix]

Nodel currently lacks any guidance on contribution. I propose adding one to guide potential contributors and streamline the contribution process.

The document should include:

• Brief technical introduction to Nodel
• Setup instructions for the development environment
• Contribution guidelines (coding conventions, commit messages, PRs)
• Issue reporting instructions

There are some really blue sky examples from large organisations:

• https://docs.djangoproject.com/en/dev/internals/contributing/
• https://github.com/kubernetes/community/blob/master/contributors/guide/README.md

But even some good examples from smaller projects:

• https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md
• https://github.com/spf13/cobra/blob/main/CONTRIBUTING.md