Add release note.

Author: twstokes

RELEASE-NOTES.txt

@@ -6,6 +6,7 @@
 * [***] You can now migrate your site content to the Jetpack app without a hitch. [#19759]
 * [**] [internal] Upgrade React Native from 0.66.2 to 0.69.4 [https://github.com/wordpress-mobile/gutenberg-mobile/pull/5193]
 * [*] [internal] When a user migrates to the Jetpack app and allows notifications, WordPress app notifications are disabled. [#19616, #19611, #19590]
+* [*] Reader now scrolls to the top if the tab bar button is tapped. [#19769]
 
 21.3
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