docs: add release notes

Author: ellemouton

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

@@ -189,7 +189,17 @@ The underlying functionality between those two options remain the same.
   it with an external graph source rather than requiring it to first sync its 
   own graph. A new `--gossip.no-sync` config option has also been added which 
   can be used to switch off gossip syncing so that LND does not attempt to sync 
-  its own graph. 
+  its own graph.
+
+* [Add an RPC implementation of the `GraphSource` 
+  interface](https://github.com/lightningnetwork/lnd/pull/9265). With this PR, 
+  users can now provide an external graph source to LND using the new 
+  `--remotegraph` config options. These can be combined with the 
+  `--gossip.no-sync` option to run a node that does not sync gossip and uses a 
+  remote LND node to provide it with network gossip data. If this configuration 
+  is being used, it is recommended to set the 
+  `--caches.rpc-graph-cache-duration` to a non-zero duration (zero is the 
+  default) so that certain calls to the remote node are cached for some time.
 
 ## Tooling and Documentation