Add changelog entry about potentialDupesIgnoringUsername

CHANGES_UNRELEASED.md

@@ -10,3 +10,10 @@
 
 - iOS: The `reqwest` network stack will not be initialized automatically anymore.
 Please call `Viaduct.shared.useReqwestBackend()` as soon as possible before using the framework. ([#2880](https://github.com/mozilla/application-services/pull/2880))
+
+## Logins
+
+### What's New
+
+- A new function was added to return a list of duplicate logins, ignoring
+  username. ([#2542](https://github.com/mozilla/application-services/pull/2542))