Add tests coverage report badge to README

## What does this MR do?

This MR adds tests coverage badge to project's README.

The job it reads coverage value from is `coverage`, and it is configured in out CI test suite to calculate compound coverage from all test jobs running in parallel.

This will pick the latest successful pipeline to get `coverage` job and coverage value.

For now, this will report test coverage only for backend test suite (Ruby).

See merge request !5925