docs: add release-notes

Author: ziggie1984

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

@@ -66,6 +66,8 @@
     database](https://github.com/lightningnetwork/lnd/pull/9147)
   * Implement query methods (QueryPayments,FetchPayment) for the [payments db 
     SQL Backend](https://github.com/lightningnetwork/lnd/pull/10287)
+  * Implement insert methods for the [payments db 
+    SQL Backend](https://github.com/lightningnetwork/lnd/pull/10291)
 
 ## Code Health