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
常用 的有：
操作系统或编辑器相关的全局模板
全局的模板：就是每一个git库都用应用到
如创建一个文件~/.gitignore_global
然后运行
git config --global core.excludesfile ~/.gitignore_global
常用 的有：
Some common .gitignore configurations
# Compiled source #
###################
*.com
*.class
*.dll
*.exe
*.o
*.so
# Packages #
############
# it's better to unpack these files and commit the raw source
# git has its own built in compression methods
*.7z
*.dmg
*.gz
*.iso
*.jar
*.rar
*.tar
*.zip
# Logs and databases #
######################
*.log
*.sql
*.sqlite
# OS generated files #
######################
.DS_Store
.DS_Store?
._*
.Spotlight-V100
.Trashes
ehthumbs.db
Thumbs.db
