At the bottom of the README there currently is...

To have only specific countries in the css file, remove the ones that you don't need from the _flag-icons-list.scss file and build it again.

However with a recent update to the SASS the list is now in _variables.scss.

This could be fixed by simply changing the filename in the README, however previously the filename reflected the list being there. I think the fix should be such that the same applies again.