doc: use consistent markdown in README

Improve markdown practices in README: * consistent header indicators * consistent emphasis indicators * wrap bare URLs in `<` and `>` * wrap at 80 chars * specifying language in fenced code PR-URL: #7971 Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com> Reviewed-By: Michaël Zasso <mic.besace@gmail.com> Reviewed-By: James M Snell <jasnell@gmail.com>

Author: Trott