Add imposm3 mapping for boundary

layers/boundary/boundary.yaml

@@ -1,12 +1,19 @@
 layer:
   id: "boundary"
-  description: Buildings
+  description: |
+    Contains [administrative boundaries](http://wiki.openstreetmap.org/wiki/Tag:boundary%3Dadministrative)
+    (no maritime boundaries yet).
+    Until z7 Natural Earth data is used which switches to OSM boundaries after z8.
   buffer_size: 4
   fields:
-    admin_level: Number
-    class: String
-    scalerank: Number
+    admin_level: |
+        OSM [admin_level](http://wiki.openstreetmap.org/wiki/Tag:boundary%3Dadministrative#admin_level)
+        indicating the level of importance of this boundary.
+        At low zoom levels the Natural Earth boundaries are mapped to the equivalent admin levels.
   datasource:
     query: (SELECT * FROM layer_boundary(!bbox!, z(!scale_denominator!))) AS t
 schema:
   - ./boundary.sql
+datasources:
+  - type: imposm3
+    mapping_file: ./mapping.yaml