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:
在空目錄下創建.gitignore文件。
文件內寫入如下代碼，可以排除空目錄下所有文件被跟蹤：
# Ignore everything in this directory
*
# Except this file !.gitignore