upgraded README, now we only support Emacs 24+
upgraded README, now we only support Emacs 24+

zuoshifan/emacs.d,zuoshifan/emacs.d,zuoshifan/emacs.d,zuoshifan/emacs.d,zuoshifan/emacs.d,zuoshifan/emacs.d,zuoshifan/emacs.d,zuoshifan/emacs.d,YangXin/emacs.d,YangXin/emacs.d,zuoshifan/emacs.d