Improve README
Ongoing issue to improve README after its initial update (#9 (closed)):
- Add badges: https://naereen.github.io/badges/
- Link to CoC and add badge: https://www.contributor-covenant.org/
[![Contributor Covenant](https://img.shields.io/badge/Contributor%20Covenant-v1.4%20adopted-ff69b4.svg)](code-of-conduct.md)
- Implement as much as possible from https://opensource.guide/
- Notes: