Document extending scripted classes in scripts

assets/content/cookbook/Advanced/02.ScriptedClasses.md

@@ -20,6 +20,54 @@ class BallisticSong extends Song {
 }
 ```
 
+## Extending Scripted Classes
+
+Since update `0.8.2`, a scripted class can also extend *another scripted class*, rather than only a native scriptable class. This is useful for sharing behavior between multiple scripted classes, like a base character that several custom characters all build on.
+
+```haxe
+// MyBaseChar.hxc
+// A scripted class that extends a native class, as usual.
+import funkin.play.character.SparrowCharacter;
+
+class MyBaseChar extends SparrowCharacter {
+  public function new() {
+    super();
+  }
+
+  // Shared behavior for all characters based on this script.
+  function onDance() {
+    // ...
+  }
+}
+```
+
+```haxe
+// MyChar.hxc
+// This scripted class extends the scripted class above.
+import MyBaseChar;
+
+class MyChar extends MyBaseChar {
+  public function new() {
+    // The superclass is itself a scripted class; you still call its constructor.
+    super();
+  }
+
+  override function onDance() {
+    // Call the inherited scripted function first.
+    super.onDance();
+    // Then add custom behavior.
+    // ...
+  }
+}
+```
+
+There are a few rules to keep in mind:
+
+- If your class defines a `new()` constructor, you must call `super()` inside it, just like with a native superclass. If you don't define a constructor at all, the superclass's constructor is called automatically.
+- A scripted class can't have an instance field with the same name as a field in its superclass — Polymod throws an error on that conflict.
+- Scripted functions and fields are inherited through the entire chain of scripted superclasses, so you can extend a scripted class that itself extends another scripted class.
+- Extending a native scriptable class (as shown in the example above) still works exactly as before.
+
 ## List of Scriptable Classes
 
 There is a predefined list of classes which the game has set up to be scriptable, and will automatically load and execute when relevant. More of these will be added in the future.