WIP: Coverage badge

What does this MR do?

This adds the ability to have a coverage badge (similar to shields.io)

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

So people can have a coverage badge in addition to build badge.

What are the relevant issue numbers?

#3714 (closed)

Screenshots (if relevant)

Before:

After:

TODO:

• Tests
• Update Changelog