improve README file
improve README file

ruchee/vimrc,ruchee/vimrc,ruchee/vimrc,ruchee/vimrc,ruchee/vimrc,ruchee/vimrc,ruchee/vimrc,ruchee/vimrc,ruchee/vimrc,ruchee/vimrc,ruchee/vimrc,ruchee/vimrc,ruchee/vimrc