Release Notes: add new section for next version (21.5)

Author: wpmobilebot

RELEASE-NOTES.txt

@@ -1,5 +1,9 @@
 *** 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).
+21.5
+-----
+
+
 21.4
 -----
 - [**] Store Picker: Sites can now be hidden from the list for a more focused experience for agencies. [https://github.com/woocommerce/woocommerce-ios/pull/14780]