[Firestore] Add release note for #11815 (#11819)

Firestore/CHANGELOG.md

@@ -1,3 +1,8 @@
+# Unreleased
+- [fixed] Fixed an issue where Firestore's binary SwiftPM distribution would
+  not link properly when building a target for testing. This issue affected
+  Xcode 15 Beta 5 and later (#11656).
+
 # 10.15.0
 - [feature] Add the option to allow the SDK to create cache indexes automatically to
   improve query execution locally. (#11596)