SubmittingPatches: clarify GitHub visual

GitHub has two general forms for its states, sometimes they're a simple colored object (e.g. green check or red x), and sometimes there's also a colored container (e.g. green box or red circle) which contains that object (e.g. check or x). That's a lot of words to try to describe things, but in general, the key for a failure is that it's recognized as an `x` and that it's associated with the color red -- the color of course is problematic for people who are red-green color-blind, but that's why they are paired with distinct shapes. Signed-off-by: Josh Soref <jsoref@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>