Ah here it is: https://www.sketch.com/docs/libraries/#sharing-a-library
Inside the XML file, you’ll need to edit the following:
<title> — The Library’s name that you want to show in Sketch
<image><url> — The default thumbnail URL that Sketch will attempt to use for your library. It should point to a publicly accessible image URL.
<item> — The information between the item tags is what you’ll want to update when you create new versions of your library. You only need one item tag within your feed.
In the the item tag, for each update you’ll need to include:
<pubDate> — The date the library update was published in the RFC822 format.
<enclosure> — The enclosure tag has three attributes: url, type & sparkle:version. The URL should point directly to your Library document’s server location, type should be “application/octet-stream” and finally sparkle:version is a number that should increment with each new update.
Make sure to serve both your XML feed and your Sketch file using https URLs.
Finally, use a tool like the Meyerweb URL encoder to encode your XML feed URL and add it on to the following:
Enter the full URl (from sketch:// through to the end your encoded URL) into your browser to open Sketch and add your shared Library.
Key thing here: Be sure to increment the sparkle version with each update.