As user, I'd like the proper badges added in my Readme when using `use_gitlab_ci()`

[statnmap]

As user, I'd like the proper badges added in my Readme when using use_gitlab_ci(), so that I can know if my CI run properly, and I get see the code coverage when working with R packages

Validation

• A pipeline status badge exists in the Readme for a R package, a golem and a book project
• A coverage status badge exists in the Readme for a R package, and a golem project

Tech

• See : gitlab-ci: Add badges in the README when running use_gitlab_ci() gitlabr#50
• At some point, this CI implementation, templates and testing should finish in an independant R package.