Add missing changlog entry for 737c9924

Add entry to changelog noting that the clusterioMod plugin has been
renamed to subspace_storage.

CHANGELOG.md

@@ -158,6 +158,7 @@ Version 2.0.0
   arguments from the master server.
 - Implemented a new config system that replaces the old.  Breaks all plugins.
 - Removed usage of socket.io entirely in favor of a plain WebSocket connection.
+- Renamed clusterioMod plugin to subspace_storage.
 
 
 Version 1.2.4