update documentation

Author: web-flow

docs/lnd/release-notes/release-notes-0.19.0.md

@@ -319,6 +319,9 @@ The underlying functionality between those two options remain the same.
 
 * [Fixed](https://github.com/lightningnetwork/lnd/pull/9549) a long standing
   unit test flake found in the `chainntnfs/bitcoindnotify` package.
+ 
+* Add a new CI-step to do some basic [backwards compatibility 
+  testing](https://github.com/lightningnetwork/lnd/pull/9540) for each PR. 
 
 ## Database