Document the co-op campaign

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

docs/CAMPAIGN_EDITOR.md

@@ -33,6 +33,7 @@ the file format in `src/campaign_doc.rs`, the scripts in `src/blocks.rs` and `sr
 
 - a `.dccamp` file (or drop the file onto the window),
 - **Open the demo** – the demo campaign, which uses every trigger kind and every special script,
+- **League against the Lightwatch** – the built-in 20-mission co-op campaign, as a copy to change or learn from,
 - **a mission of the built-in campaign as a copy** – its map as it starts, its texts and goals,
   added to the current campaign (the original stays untouched),
 - **my campaigns (online)** and **published campaigns** of all players (needs a login on a server