Add version number to device-flow note.

Author: tarickb

README.md

@@ -288,8 +288,8 @@ Skip to [restart Postfix](#restart-postfix) below.
 
 ### Outlook/Office 365 Configuration (Device Flow)
 
-This is the preferred method to authenticate with Outlook/Office, but the
-[fallback legacy client
+As of sasl-xoauth2-0.23, this is the preferred method to authenticate with
+Outlook/Office, but the [fallback legacy client
 approach](#outlookoffice-365-configuration-legacy-client-deprecated) does still
 work (... for now).