How to let our users know about our DLC?

We are releasing episodic content, which is purchasable as IAPs through the app itself. We launch a month or so ago with the first episode, and have now released the second episode, which users can buy through the menu inside the experience.

However, most of our users will have downloaded our app, used it, and then either maybe uninstalled it (since even if they enjoyed it, they might not plan on doing it again), or it will just sit there unopened again for some time.

So my question is: how can we contact our user base to let them know that the second episode is available? I understand that being able to message out to all the people who downloaded an app could lead to some serious spamming, but I'm sure there must be something we can do?!