add readme of /doc 
add readme of /doc 

in order to announce readers and developers that the documentation source of PaddlePaddle.org has been moved to Fluiddoc repo
chengduoZH/Paddle,luotao1/Paddle,luotao1/Paddle,chengduoZH/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,reyoung/Paddle,reyoung/Paddle,tensor-tang/Paddle,chengduoZH/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,luotao1/Paddle,baidu/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,baidu/Paddle,baidu/Paddle,baidu/Paddle,tensor-tang/Paddle,reyoung/Paddle,reyoung/Paddle,luotao1/Paddle,tensor-tang/Paddle,baidu/Paddle,chengduoZH/Paddle,reyoung/Paddle,luotao1/Paddle,reyoung/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,chengduoZH/Paddle,luotao1/Paddle