Broken badge on README

[BenMorel]

Since #53, the CI badge on the README is broken: https://github.com/brick/date-time/workflows/CI/badge.svg

This is because we don't have a single CI workflow anymore, but 3 workflows:

• Coding Style
• Static Analysis
• Tests

Each having its own badge:

• https://github.com/brick/date-time/workflows/Coding%20Style/badge.svg
• https://github.com/brick/date-time/workflows/Static%20Analysis/badge.svg
• https://github.com/brick/date-time/workflows/Tests/badge.svg

@tigitz Do you know of a way to get a single badge for all 3 workflows?

[tigitz]

AFAIU, there doesn't seem to be a way to aggregate those statuses into a single badge, no.

Unless we put all the jobs into a single "CI" workflow and generate a badge from this.