Add name_de to documentation

layers/water_name/water_name.yaml

@@ -6,7 +6,8 @@ layer:
       which derives nice centerlines from OSM water bodies. Only the most important lakes contain labels.
   fields:
     name: The OSM [`name`](http://wiki.openstreetmap.org/wiki/Key:name) value of the water body.
-    name_en: The english `name:en` value if available.
+    name_en: English name `name:en` if available, otherwise `name`.
+    name_de: German name `name:de` if available, otherwise `name` or `name:en`.
     class:
       description: |
           At the moment only `lake` since no ocean parts are labelled. *Reserved for future use*.