Adding install instructions for ftxvalidator on MacOS
Adding install instructions for ftxvalidator on MacOS

(issue #1927)

moyogo/fontbakery,graphicore/fontbakery,graphicore/fontbakery,moyogo/fontbakery,moyogo/fontbakery,googlefonts/fontbakery,graphicore/fontbakery,googlefonts/fontbakery,googlefonts/fontbakery