Document landcover files

README.md

@@ -35,6 +35,8 @@ tilemaker comes with configuration files compatible with the popular [OpenMapTil
 
 If you want to include sea tiles, then create a directory called `coastline` in the same place you're running tilemaker from, and then save the files from https://osmdata.openstreetmap.de/download/water-polygons-split-4326.zip in it, such that tilemaker can find a file at `coastline/water_polygons.shp`.
 
+_(If you want to include optional small-scale landcover, create a `landcover` directory, and download the appropriate 10m files from 'Features' at https://www.naturalearthdata.com so that you have `landcover/ne_10m_antarctic_ice_shelves_polys/ne_10m_antarctic_ice_shelves_polys.shp`, `landcover/ne_10m_urban_areas/ne_10m_urban_areas.shp`, `landcover/ne_10m_glaciated_areas/ne_10m_glaciated_areas.shp`.)_
+
 Then, to serve your tiles using the demonstration server:
 
     cd server