add_github_actions_check_badge: Add a R CMD Check badge
Description
This function adds a R CMD Check badge to the README.Rmd. This function
must be run after add_github_actions_check() which will setup GitHub
Actions to check and test the package.
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 -->.