Mention PR #7439 in CHANGELOG.md.

CHANGELOG.md

@@ -5,6 +5,9 @@ TBD
 
 ### Improvements
 
+- `InMemoryCache` now _guarantees_ that any two result objects returned by the cache (from `readQuery`, `readFragment`, etc.) will be referentially equal (`===`) if they are deeply equal. Previously, `===` equality was often achievable for results for the same query, on a best-effort basis. Now, equivalent result objects will be automatically shared among the result trees of completely different queries. This guarantee is important for taking full advantage of optimistic updates that correctly guess the final data, and for "pure" UI components that can skip re-rendering when their input data are unchanged. <br/>
+  [@benjamn](https://github.com/benjamn) in [#7439](https://github.com/apollographql/apollo-client/pull/7439)
+
 - Support `client.refetchQueries` as an imperative way to refetch queries, without having to pass `options.refetchQueries` to `client.mutate`. <br/>
   [@dannycochran](https://github.com/dannycochran) in [#7431](https://github.com/apollographql/apollo-client/pull/7431)