Add release notes.

Author: twstokes

RELEASE-NOTES.txt

@@ -4,6 +4,7 @@
 
 21.3
 -----
+* [***] Adds a smooth, opt-in transition to the Jetpack app. [#19714]
 * [*] [internal] When a user migrates to the Jetpack app and allows notifications, WordPress app notifications are disabled. This is released disabled and is behind a feature flag. [#19616, #19611, #19590]
 * [*] Fixed a minor UI issue where the segmented control under My SIte was being clipped when "Home" is selected. [#19595]
 * [*] Fixed an issue where the site wasn't removed and the app wasn't refreshed after disconnecting the site from WordPress.com. [#19634]