Release Notes: add new section for next version (21.9)

Author: wpmobilebot

RELEASE-NOTES.txt

@@ -1,6 +1,10 @@
 *** PLEASE FOLLOW THIS FORMAT: [<priority indicator, more stars = higher priority>] <description> [<PR URL>]
 *** Use [*****] to indicate smoke tests of all critical flows should be run on the final IPA before release (e.g. major library or OS update).
 
+21.9
+-----
+
+
 21.8
 -----
 - [**] Shipping Labels: resolved issue preventing shipping labels from displaying in some orders when an error occurred during label creation. [https://github.com/woocommerce/woocommerce-ios/pull/15101]