Release notes

Author: var-const

release_build_files/readme.md

@@ -574,6 +574,8 @@ code.
         the overload that takes a `std::function` argument instead.
     -   Firestore: `FieldValue::Increment` functions are no longer guarded by
         the `INTERNAL_EXPERIMENTAL` macro.
+    -   Firestore: add an `is_valid` method to the public API classes that can
+        be in an invalid state.
 
 ### 8.2.0
 -   Changes