This function adds a Code coverage badge to the README.Rmd, i.e. the
percentage of code cover by units tests. This percentage is computed by
the codecov.io service.
Note: this service must be manually activated for the package by visiting
https://about.codecov.io/.
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 -->.