setup global gitignore to ignore temporary files

.gitignore file in a rails project says:

If you find yourself ignoring temporary files generated by your text editor
or operating system, you probably want to add a global ignore instead:
git config --global core.excludesfile '~/.gitignore_global'

https://help.github.com/articles/ignoring-files
https://github.com/github/gitignore/blob/master/Ruby.gitignore
https://github.com/github/gitignore/blob/master/Rails.gitignore

this is good sometime but not always