git clone <repository> --recursive 递归的方式克隆整个项目
git submodule add <repository> <path> 添加子模块
git submodule init 初始化子模块
git submodule update 更新子模块
git submodule foreach git pull 拉取所有子模块

project
 |--moduleA
 |--readme.txt

git init --bare project.git
git clone project.git project1
cd project1
echo "This is a project." > readme.txt
git add .
git commit -m "add readme.txt"
git push origin master
cd ..

git init --bare moduleA.git
git clone moduleA.git moduleA1
cd moduleA1
echo "This is a submodule." > a.txt
git add .
git commit -m "add a.txt"
git push origin master
cd ..

cd project1
git submodule add ../moduleA.git moduleA
git status
git diff
git add .
git commit -m "add submodule"
git push origin master
cd ..

On branch master
Your branch is up-to-date with 'origin/master'.
Changes to be committed:
 (use "git reset HEAD <file>..." to unstage)
 new file:  .gitmodules
 new file:  moduleA

git clone project.git project2
cd project2
git submodule init
git submodule update
cd ..

git clone project.git project3 --recursive

cd project1/moduleA
git branch
echo "This is a submodule." > b.txt
git add .
git commit -m "add b.txt"
git push origin master
cd ..
git status
git diff
git add .
git commit -m "update submodule add b.txt"
git push origin master
cd ..

cd project2
git pull
git submodule update
cd ..

cd project3/moduleA
git checkout master
cd ..
git submodule foreach git pull
cd ..

git rm --cached moduleA
rm -rf moduleA
rm .gitmodules
vim .git/config

[submodule "moduleA"]
   url = /Users/nick/dev/nick-doc/testGitSubmodule/moduleA.git

git add .
git commit -m "remove submodule"

git rm moduleA
git status
git commit -m "remove submodule"
git push origin master