doc: update release notes

Author: morehouse

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

@@ -73,6 +73,8 @@
 * [Fixed a bug in the `SignPsbt` RPC that produced an invalid response when
   signing a NP2WKH input](https://github.com/lightningnetwork/lnd/pull/6687).
 
+* [Fix race condition in `sign_psbt` test](https://github.com/lightningnetwork/lnd/pull/6741).
+
 * [Update the `urfave/cli`
   package](https://github.com/lightningnetwork/lnd/pull/6682) because of a flag
   parsing bug.
@@ -116,6 +118,7 @@
 * Elle Mouton
 * ErikEk
 * Eugene Siegel
+* Matt Morehouse
 * Slyghtning
 * Oliver Gugger
 * Olaoluwa Osuntokun