Co-op matches of editor campaigns

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/CAMPAIGN_EDITOR.md

@@ -116,6 +116,9 @@ Ctrl+Z/Y undo/redo, Ctrl+C/V copy/paste the selected block, Delete removes it, C
 - **Online** (server with the savegame service): own campaigns per user, overwrite on the next
   save; *Published* adds the campaign to the list every logged-in player sees and can open (as a
   copy) and play. Server API: `/api/campaigns`, `/api/public/campaigns` (see `server/src/main.rs`).
+- **Co-op**: the campaign last opened in the editor can be played in a co-op match (Multiplayer,
+  *Editor campaign*), one mission per match; missions need a map of at least 72x72 and a goal besides
+  rival masters. See `docs/MULTIPLAYER.md`.
 
 ## File format (`src/campaign_doc.rs`)