Add a badge for CI status

[EdZou]

Description:

Is your feature request related to a problem?
Most of the existing open-telemetry repositories (java/javascript/python/collector/go/ruby/rust) display a badge for CI status. It would be very useful to have a CI badge for the CPP repo too.

Describe the solution you'd like.
As a developer contributing to open-telemetry, I recommend to add a CI status badge on the README.md of C++ repo. Because CI can increases the speed of development while ensuring the quality of the code that teams deploy.
And by adding a badge to README.md, a developer will be able to know CI status easily by access CI badge.

Describe alternatives you've considered.
As seen in the Rust repo, we may provide a CI status badge which links to “Actions” page instead of linking to CircleCI website as well.

[pyohannes]

This is covered by #149.