update changelog (#692)

Author: SahilAujla

fern/changelog/2025-09-18.md

@@ -1,6 +1,11 @@
 ## Developer Experience
 
-Redesigned dashboard to streamline navigation between apps:
+**Custom usage alerts are back.**
+
+* Existing usage alerts have been re-enabled.
+* Pay-as-you-go and Enterprise developers are now able to create new usage alerts by setting usage thresholds in CUs or dollars.
+
+**Redesigned dashboard to streamline navigation between apps:**
 
 * Select an app to see its metrics, services (node, wallets, rollups, webhooks, etc.), and settings.
 * Easily switch between apps with the new header.