Add rationale to check/008 (consistent underline thickness)
Add rationale to check/008 (consistent underline thickness)

(issue #1692)

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