Add Changelog for Query.get()

Author: schmidt-sebastian

firebase-database/CHANGELOG.md

@@ -1,6 +1,9 @@
 # Unreleased
 - [fixed] Fixed a crash on some Pixel devices that occurred when closing the
   network connection.
+- [added] Added `Query.get()`, which allows users to receive a single data
+  snapshot. `Query.get()` returns the latest value even if an older value
+  already exists in cache.
 
 # 19.5.1
 - [fixed] Fixes a regression in v19.4 that may cause assertion failures,