Update CHANGELOG.md

Author: daymxn

firebase-ai/CHANGELOG.md

@@ -5,6 +5,9 @@
   * The new Firebase AI SDK provides **public preview** support for the Gemini Developer API, including its free tier offering.
   * Using the Firebase AI SDK with the Vertex AI Gemini API is still generally available (GA).
 * [feature] **Public Preview:** Added support for specifying response modalities in GenerationConfig
- (only available in the new `firebase-ai` package). This includes public experimental support for
- image generation using specific Gemini models; for details, see https://firebase.google.com/docs/vertex-ai/models.
+ (only available in the new `firebase-ai` package). This includes support for image generation using
+ specific Gemini models; for details, see https://firebase.google.com/docs/vertex-ai/models.
+
+ Note: This feature is in Public Preview and relies on experimental models, which means that it is not subject to any SLA or
+ deprecation policy and could change in backwards-incompatible ways.