docs: add release-notes

Author: ziggie1984

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

@@ -68,6 +68,9 @@
     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)
+  * Implement third(final) Part of SQL backend [payment functions]
+  (https://github.com/lightningnetwork/lnd/pull/10368)
+
 
 ## Code Health