Document that other JVM languages are not fully supported (#2666)

Author: Marcono1234

README.md

@@ -8,6 +8,10 @@ There are a few open-source projects that can convert Java objects to JSON. Howe
 > [!NOTE]\
 > Gson is currently in maintenance mode; existing bugs will be fixed, but large new features will likely not be added. If you want to add a new feature, please first search for existing GitHub issues, or create a new one to discuss the feature and get feedback.
 
+> [!IMPORTANT]\
+> Gson's main focus is on Java. Using it with other JVM languages such as Kotlin or Scala might work fine in many cases, but language-specific features such as Kotlin's non-`null` types or constructors with default arguments are not supported. This can lead to confusing and incorrect behavior.\
+> When using languages other than Java, prefer a JSON library with explicit support for that language.
+
 ### Goals
   * Provide simple `toJson()` and `fromJson()` methods to convert Java objects to JSON and vice-versa
   * Allow pre-existing unmodifiable objects to be converted to and from JSON