作用是让临时文件和中间文件都不提交到代码库中
工程相关的. gitignore 放在根目录
https://github.com/github/gitignore/blob/master/Android.gitignore
https://github.com/github/gitignore/blob/master/C%2B%2B.gitignore
https://github.com/github/gitignore/blob/master/C.gitignore
https://github.com/github/gitignore/blob/master/Python.gitignore
https://github.com/github/gitignore/blob/master/VisualStudio.gitignore
