Add docs for landcover, landuse and housenumber

layers/landcover/README.md

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+# landcover
+
+Landcover is used to describe the physical material at the surface of the earth. At lower zoom levels this is
+from Natural Earth data for glaciers and ice shelves and at higher zoom levels the landcover is implied
+by OSM tags (like `landuse` or `natural`).
+
+## Fields
+
+- **class**: More generalized groups of either `farmland`, `ice`, `wood`, `grass` or `wetland`.
+Use this to assign general colors to the landcover.
+- **subclass**: Original value of either the `landuse`, `natural` or `wetland` tag.
+Use this to do more precise styling.
+
+## Mapping
+
+![](mapping.png)
+
+