Update changelog for v0.15.0.

Author: var-const

Firestore/CHANGELOG.md

@@ -1,21 +1,22 @@
 # Unreleased
 
+# v0.15.0
 - [changed] Changed how the SDK handles locally-updated documents while syncing
   those updates with Cloud Firestore servers. This can lead to slight behavior
   changes and may affect the `SnapshotMetadata.hasPendingWrites` metadata flag.
 - [changed] Eliminated superfluous update events for locally cached documents
   that are known to lag behind the server version. Instead, the SDK buffers
   these events until the client has caught up with the server.
-
-# v0.14.0
-- [fixed] Fixed compilation in C99 and C++11 modes without GNU extensions.
 - [changed] Moved from Objective-C gRPC framework to gRPC C++. If you're
   manually tracking dependencies, the `gRPC`, `gRPC-ProtoRPC`, and
   `gRPC-RxLibrary` frameworks have been replaced with `gRPC-C++`. While we
   don't anticipate any issues, please [report any issues with network
   behavior](https://github.com/firebase/firebase-ios-sdk/issues/new) you
   experience. (#1968)
 
+# v0.14.0
+- [fixed] Fixed compilation in C99 and C++11 modes without GNU extensions.
+
 # v0.13.6
 - [changed] Internal improvements.