http://stackoverflow.com/questions/115983/how-do-i-add-an-empty-directory-to-a-git-repository
Another way to make a directory stay empty (in the repo) is to create a .gitignore inside that directory that contains two lines:
在空目录下创建.gitignore文件。
文件内写入如下代码，可以排除空目录下所有文件被跟踪：
# Ignore everything in this directory
*
# Except this file !.gitignore