Add Ma Chao retreat scenario

data/campaign/campaign.json

@@ -188,6 +188,11 @@
       "id": "037_guanzhong_foothold",
       "title": "Guanzhong Foothold",
       "path": "res://data/scenarios/037_guanzhong_foothold.json"
+    },
+    {
+      "id": "038_ma_chao_retreat",
+      "title": "Ma Chao Retreat",
+      "path": "res://data/scenarios/038_ma_chao_retreat.json"
     }
   ]
 }