This function adds a CRAN Status badge to the README.Rmd. If the
package is not hosted on the CRAN the badge will indicate
not published on the CRAN.
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 -->.
Don't forget to re-render the README.md.
