This is a feature that I find invaluable, but that I don't recall seeing a huge amount of documentation on at the time I started using it. (Turns o...
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To unignore something, add it to the local .gitignore file with a leading
!Be careful with this. Sometimes when you're setting up new projects those files are already ignored, but won't be ignored for anyone else unless they also have a global ignore like yours. (And vice-versa and thing not meant to be ignored will be ignored)
Better to do on a per project basis
According to the gitignore man page, the "default value [for core.excludesfile] is $XDG_CONFIG_HOME/git/ignore. If $XDG_CONFIG_HOME is either not set or empty, $HOME/.config/git/ignore is used instead."
git-scm.com/docs/gitignore