Add maintenance mode notice (#26)

src/1-cardinalkit-app/README.md

@@ -1,5 +1,9 @@
 # CardinalKit iOS app
 
+::: tip
+The CardinalKit iOS template application is in maintenance mode as of July 2023. All new projects should use [Stanford Spezi](https://spezi.stanford.edu), our modular and standards-based framework successor to CardinalKit. Check out the [Spezi Template Application](https://github.com/StanfordSpezi/SpeziTemplateApplication) as a great way to get started.
+:::
+
 In this section, we will guide you through setting up a new CardinalKit iOS app using the open-source template available on our Github. You will need a Macintosh computer.
 
 Go to [Create a New CardinalKit iOS App](/cardinalkit-docs/1-cardinalkit-app/1-start.html) to get started.