add_github_actions_codecov_badge: Add a Test coverage badge
Description
This function adds a Test coverage badge to the README.Rmd. This
function must be run after add_github_actions_codecov() which will setup
GitHub Actions to report the percentage of code cover by units tests.
Make sure that 1) a README.Rmd file exists at the project root and 2) it
contains a block starting with the line <!-- badges: start --> and ending
with the line <!-- badges: end -->.