Update: release notes

Author: Momo Ozawa

RELEASE-NOTES.txt

@@ -2,6 +2,7 @@
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 * [**] Some of the screens of the app has a new, fresh and more modern visual, including the initial one: My Site. [#17812]
 * [**] Notifications: added a button to mark all notifications in the selected filter as read. [#17840]
+* [**] Quick Start: Fixed a bug where a user logging in via a self-hosted site not connected to Jetpack would see Quick Start when selecting "No thanks" on the Quick Start prompt. [#17855] 
 
 19.1
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