update doc to mention rotation about centre
update doc to mention rotation about centre

menpo/menpo,patricksnape/menpo,yuxiang-zhou/menpo,grigorisg9gr/menpo,grigorisg9gr/menpo,menpo/menpo,yuxiang-zhou/menpo,yuxiang-zhou/menpo,grigorisg9gr/menpo,patricksnape/menpo,menpo/menpo,patricksnape/menpo