gitignore_global is a great idea. I had not heard of it before. 

But here it is: https://help.github.com/articles/ignoring-files/  . This instruction also has a link to a gist with a lot of helpful global ignores.

I understand your reluctance to add entries for every IDE. Maybe a better solution would be to add something to the devguide about excluding IDE settings files, with a link to the Github instructions.