You can simply add a .gitignore file to your home directory, i.e. $HOME/.gitignore
or ~/.gitignore
. Then tell git to use that file with the command:
git config --global core.excludesfile ~/.gitignore
This is a normal .gitignore file which git references when deciding what to ignore. Since it's in your home directory, it applies only to you and doesn't pollute any project .gitignore files.
I've been using this approach for years with great results.