Add changelog entry

rs/nns/governance/unreleased_changelog.md

@@ -37,6 +37,17 @@ In this relesae, we turn on 2 features related to migrating active neurons to st
 
 No neurons are actually migrated yet.
 
+### List Neurons API Change: Query by Subaccount
+
+The `list_neurons` API now supports querying by neuron subaccount.  This is useful for neuron holders who
+have many neurons and want to list only the neurons associated with a particular subaccount.
+
+A new field `neuron_subaccounts` is added to the request, which is a list of subaccounts to query
+for.  If this field is present, any neurons found will be added to the response.  If duplicate
+neurons are found between this field and others, they will be deduplicated before returning the value.
+
+This new field works in the same way that the existing `neuron_ids` field works.
+
 ## Changed
 
 * `InstallCode` proposal payload hashes are now computed when making the proposal instead of when