[DNMY] Add conventions to README

[euronion]

I think we should write down a few conventions and try to make the repository comply to them as far as possible

I have added a few ideas, happy to receive feedback.

Checklist

• [ ] Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• [ ] Data source for new technologies is cleary stated.
• [ ] Newly introduced dependencies are added to environment.yaml (if applicable).
• [ ] A note for the release notes doc/release_notes.rst of the upcoming release is included.
• [ ] I consent to the release of this PR's code under the GPLv3 license.

[energyLS]

@energyls request for comment (DNMY: Do not merge yet)