docs: Adds Charmhub badge

[gruyaume]

Description

The Telco charms currently have broken statuses in the charm engineering page. This PR fixes this issues and brings the README.md in line with the other team's (ex. Grafana K8s Operator).

Checklist:

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that validate the behaviour of the software
• [ ] I validated that new and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules
• [ ] I have bumped the version of the library