Mention PR #7070 in CHANGELOG.md.

CHANGELOG.md

@@ -9,6 +9,9 @@
 - Support inheritance of type and field policies, according to `possibleTypes`. <br/>
   [@benjamn](https://github.com/benjamn) in [#7065](https://github.com/apollographql/apollo-client/pull/7065)
 
+- Allow configuring custom `merge` functions, including the `merge: true` and `merge: false` shorthands, in type policies as well as field policies. <br/>
+  [@benjamn](https://github.com/benjamn) in [#7070](https://github.com/apollographql/apollo-client/pull/7070)
+
 - Shallow-merge `options.variables` when combining existing or default options with newly-provided options, so new variables do not completely overwrite existing variables. <br/>
   [@amannn](https://github.com/amannn) in [#6927](https://github.com/apollographql/apollo-client/pull/6927)