Update documents to init submodule.

Author: Xreki

doc/getstarted/build_and_install/build_from_source.md

@@ -11,6 +11,7 @@ You can download PaddlePaddle from the [github source](https://github.com/Paddle
 ```bash
 git clone https://github.com/PaddlePaddle/Paddle paddle
 cd paddle
+git submodule update --init --recursive
 ```
 
 ## <span id="requirements">Requirements</span>

doc/getstarted/build_and_install/docker_install.rst

@@ -79,6 +79,7 @@ source code:
    cd ~
    git clone github.com/PaddlePaddle/Paddle
    cd Paddle
+   git submodule update --init --recursive
    docker build --build-arg WITH_AVX=OFF -t paddle:cpu-noavx -f paddle/scripts/docker/Dockerfile .
    docker build --build-arg WITH_AVX=OFF -t paddle:gpu-noavx -f paddle/scripts/docker/Dockerfile.gpu .
 

doc/howto/contribute_to_paddle.md

@@ -36,8 +36,9 @@ If your repository doesn't contain **develop** branch, just create it by your ow
 git clone https://github.com/USERNAME/Paddle.git Paddle
 cd Paddle
 git checkout -b develop  # create develop branch.
-git remote add upstream https://github.com/baidu/Paddle.git  # add upstream to baidu/Paddle
+git remote add upstream https://github.com/PaddlePaddle/Paddle.git  # add upstream to baidu/Paddle
 git pull upstream develop  # update to upstream
+git submodule update --init --recursive
 ```
 
 Then you can start to develop by making a local developement branch
@@ -69,7 +70,7 @@ To do this, you'll need to add a remote at first:
 # see the current configured remote repository
 git remote -v
 # add upstream repository
-git remote add upstream https://github.com/baidu/Paddle.git
+git remote add upstream https://github.com/PaddlePaddle/Paddle.git
 # verify the new upstream
 git remote -v
 ```