docs: add release notes for addinvoice --qr flag

Author: hieblmi

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

@@ -141,6 +141,11 @@
 * The `encryptdebugpackage` command now supports an `--include_log` flag. When
   set, the log file content is included in the encrypted debug package.
 
+* The `addinvoice` command now supports a [`--qr`
+  flag](https://github.com/lightningnetwork/lnd/pull/10638) that renders the
+  payment request as a QR code directly in the terminal, making it easy to scan
+  from a mobile wallet.
+
 ## Breaking Changes
 
 * [Increased MinCLTVDelta from 18 to