Update Changelog for 4.0.2 release. (firebase#217)

Author: ryanwilson

Firebase/Database/CHANGELOG.md

@@ -1,3 +1,7 @@
+# v4.0.2
+- [fixed] Retrieving a Database instance for a specific `FirebaseApp` no longer returns a stale instance if that app was deleted.
+- [changed] Added message about bandwidth usage in error for queries without indexes.
+
 # v4.0.1
 - [changed] We now purge the local cache if we can't load from it.
 - [fixed] Removed implicit number type conversion for some integers that were represented as doubles after round-tripping through the server.
@@ -38,4 +42,4 @@
   observer immediately after connecting to the Firebase Database backend could
   result in completion blocks for other operations not getting executed.
 - [fixed] Fixed an issue affecting apps using offline disk persistence where
-  large integer values could lose precision after an app restart.
+  large integer values could lose precision after an app restart.