At Apple's World Wide Developers Conference (WWDC) 2018 it was announced there is now support for third-party subscriptions within Swift Playgrounds.
As someone who uses Playgrounds for coursework to match the Australian Digital Technologies Curriculum it is a welcome relief that we can now push updated Playground Books to students and they can see when an update is available.
After some initial reading of the available documentation and some reverse engineering of existing subscriptions available I managed to get my feed configured. I have since updated all of my iTunes U courses and made a GitHub Site to allow a direct link to the Playground Subscription.
If you are looking to do the same I would recommend checking out the awesome WWDC 18 video on Playground Subscriptions, presented by two of Apple's Engineers; Holly Borla (Xcode Engineer) and Grace Kendall (Playgrounds Engineer). They do a fantastic job at explaining the new features in Swift Playgrounds and explaining the process of setting up a Playground Subscription using GitHub Pages.