This plugin was a first attempt to bring version control and collaboration to designers. Since then, Sketch evolved a lot and my new project takes advantages of them to unlock true version control. Check it out: http://kactus.io
git-sketch-plugin
A Git client built right into Sketch. Generate pretty diffs so that everybody knows what are the changes!
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Requirements
Sketch >= 3.4 (not with the sandboxed version ie from the App Store).
Git (coming with OS X so you shouldn't have to do anything)