docs: update release notes for v0.18.0-beta

Author: bhandras

docs/release-notes/release-notes-0.18.0.md

@@ -327,6 +327,10 @@
 * [Refactor UpdateInvoice](https://github.com/lightningnetwork/lnd/pull/8100) to
   make it simpler to adjust code to also support SQL InvoiceDB implementation.
 
+* [InvoiceDB implementation](https://github.com/lightningnetwork/lnd/pull/8052)
+  for SQL backends enabling new users to optionally use  an experimental native
+  SQL invoices database.
+
 ## Code Health
 
 * [Remove database pointers](https://github.com/lightningnetwork/lnd/pull/8117)