add comments for options in python rnn example readme (#2454)
add comments for options in python rnn example readme (#2454)


yiheng/BigDL,zhangxiaoli73/BigDL,yiheng/BigDL,wzhongyuan/BigDL,i8run/BigDL-1,zhangxiaoli73/BigDL,wzhongyuan/BigDL,yiheng/BigDL,zhangxiaoli73/BigDL,i8run/BigDL-1,i8run/BigDL-1,wzhongyuan/BigDL