Official Beta announcement from Sketch:
Libraries will be coming soon in Sketch 47! Try it yourself with the Sketch 47 Beta and let us know what you think.
The best design happens when designers collaborate together. With Libraries, designers can now share Symbols across documents and have them update to be always kept in sync.
A Library is just an ordinary Sketch document that contains Symbols which you can then use in any other Sketch document. If you update any of those Symbols in your Library file, documents containing instances of those Symbols will receive a notification telling you that they can be updated. Here you can preview, check, and confirm changes — and by doing so, you can always ensure your documents are using the up-to-date copies of those components…