update: release notes

Author: Momo Ozawa

RELEASE-NOTES.txt

@@ -2,6 +2,7 @@
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 * [*] [Jetpack-only] Recommend App: you can now share the Jetpack app with your friends. [#19174]
 * [*] [Jetpack-only] Feature Announcements: new features are highlighted via the What's New modals. [#19176]
+* [**] [Jetpack-only] Self-hosted sites: enables logging in via a self-hosted site / adding a self-hosted site [#19194]
 * [*] Pages List: Fixed an issue where the app would freeze when opening the pages list if one of the featured images is a GIF. [#19184]