update what's new

Author: efectn

docs/whats_new.md

@@ -59,6 +59,7 @@ We have made several changes to the Fiber app, including:
 - **RegisterCustomBinder**: Allows for the registration of custom binders.
 - **RegisterCustomConstraint**: Allows for the registration of custom constraints.
 - **NewCtxFunc**: Introduces a new context function.
+- **State**: Provides a global state for the application, which can be used to store and retrieve data across the application. Check out the [State](./api/state) method for further details.
 
 ### Removed Methods