add a brief introduction of MKL-DNN work in root README.md
add a brief introduction of MKL-DNN work in root README.md

PaddlePaddle/Paddle,Canpio/Paddle,baidu/Paddle,baidu/Paddle,PaddlePaddle/Paddle,QiJune/Paddle,jacquesqiao/Paddle,reyoung/Paddle,luotao1/Paddle,jacquesqiao/Paddle,reyoung/Paddle,PaddlePaddle/Paddle,chengduoZH/Paddle,lcy-seso/Paddle,Canpio/Paddle,baidu/Paddle,luotao1/Paddle,Canpio/Paddle,QiJune/Paddle,jacquesqiao/Paddle,QiJune/Paddle,Canpio/Paddle,pkuyym/Paddle,chengduoZH/Paddle,jacquesqiao/Paddle,luotao1/Paddle,pkuyym/Paddle,tensor-tang/Paddle,reyoung/Paddle,putcn/Paddle,Canpio/Paddle,Canpio/Paddle,tensor-tang/Paddle,chengduoZH/Paddle,jacquesqiao/Paddle,QiJune/Paddle,luotao1/Paddle,putcn/Paddle,luotao1/Paddle,baidu/Paddle,chengduoZH/Paddle,reyoung/Paddle,QiJune/Paddle,putcn/Paddle,tensor-tang/Paddle,luotao1/Paddle,reyoung/Paddle,putcn/Paddle,lcy-seso/Paddle,lcy-seso/Paddle,lcy-seso/Paddle,luotao1/Paddle,pkuyym/Paddle,pkuyym/Paddle,tensor-tang/Paddle,reyoung/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,Canpio/Paddle,putcn/Paddle,chengduoZH/Paddle,Canpio/Paddle,lcy-seso/Paddle,tensor-tang/Paddle,jacquesqiao/Paddle,pkuyym/Paddle,QiJune/Paddle,putcn/Paddle,pkuyym/Paddle,lcy-seso/Paddle,baidu/Paddle