docs: add release notes

docs/release-notes/release-notes-0.15.1.md

@@ -25,6 +25,9 @@
 * Fixed data race found in
   [`TestSerializeHTLCEntries`](https://github.com/lightningnetwork/lnd/pull/6673).
 
+* [Fixed a bug in the `SignPsbt` RPC that produced an invalid response when
+  signing a NP2WKH input](https://github.com/lightningnetwork/lnd/pull/6687).
+
 ## RPC Server
 
 * [Add wallet reserve rpc & field in wallet balance](https://github.com/lightningnetwork/lnd/pull/6592)