docs: add release notes

Author: guggero

docs/release-notes/release-notes-0.15.5.md

@@ -9,6 +9,10 @@
   received](https://github.com/lightningnetwork/lnd/pull/7095) to avoid CPU
   spike.
 
+* [The wallet birthday is now used properly when creating a watch-only wallet
+  to avoid scanning the whole
+  chain](https://github.com/lightningnetwork/lnd/pull/7056).
+
 # Contributors (Alphabetical Order)
 
 * Oliver Gugger