docs: add release notes for addinvoice --qr flag

Author: hieblmi

docs/release-notes/release-notes-0.21.0.md

@@ -105,6 +105,10 @@
 * The `estimatefee` command now supports the `--utxos` flag to specify explicit
   inputs for fee estimation.
 
+* The `addinvoice` command now supports a `--qr` flag that renders the payment
+  request as a QR code directly in the terminal, making it easy to scan from a
+  mobile wallet.
+
 # Improvements
 ## Functional Updates