Add descriptions to builtin layers

assets/layers/bike_cafe/bike_cafe.json

@@ -374,5 +374,8 @@
         "render": "2"
       }
     }
-  ]
+  ],
+  "description": {
+    "en": "A bike café is a café geared towards cyclists, for example with services such as a pump, with lots of bicycle-related decoration, ..."
+  }
 }