Add localnet repository split release note (#2652)

docs/release-notes/RELEASE-NOTES-4.2.0.md

@@ -7,6 +7,7 @@ It includes the following changes:
 * Off-chain updates can now still be completed even if one of the Websocket clients is off-line.
   If the originator manages to get the initial update request co-signed 'out-of-band', the
   responding FSM will proceed as if it received a successful signing response from the client.
+* Localnet (docker-compose) configuration has ben moved out to its own repository https://github.com/aeternity/localnet
 
 [this-release]: https://github.com/aeternity/aeternity/releases/tag/v4.2.0