copy patch note from cpp 9.3 to unity 9.3 (#415)

Author: AlmostMatt

docs/readme.md

@@ -155,6 +155,13 @@ Support
 
 Release Notes
 -------------
+### 9.3.0
+- Changes
+    - General (Android,Linux): Fixed a concurrency bug where waiting for an
+      event with a timeout could occasionally return prematurely, as if the
+      timeout had occurred
+      ([#1021](https://github.com/firebase/firebase-cpp-sdk/pull/1021)).
+
 ### 9.2.0
 - Changes
     - Crashlytics: Fix requiring user code to reference Crashlytics when using il2cpp.