docs: Add register plugin section on migrate guide (#310)

docs/main/updating/6-0.md

@@ -44,6 +44,11 @@ Capacitor 6 requires Xcode 15.0+.
 
 Converting from using Cocoapods to SPM is a pretty large topic we will cover in a different article, coming soon.
 
+### Register custom plugins
+
+In Capacitor 6, plugin classes are no longer automatically registered. For npm installed plugins, the CLI will generate a list of plugin classes to register them programmatically.
+But users following the [custom code guide](../ios/custom-code.md) for creating local plugins not distributed through npm, they will have to create [a custom view controller and register their plugins](../ios/custom-code.md#register-the-plugin).
+
 ## Android
 
 The following guide describes how to upgrade your Capacitor 5 Android project to Capacitor 6.