Addons docs

doc/user/reference/arctic-dem-addon-data.md

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+# ArcticDEM add-on data
+
+[The Polar Geospatial Center](https://www.pgc.umn.edu/) provides a [custom
+Greenland 32-meter resolution
+ArcticDEM](https://data.pgc.umn.edu/gis/packages/qgreenland/packages/addons/)
+for users of QGreenland.
+
+Two download packages are provided for the v4.1 ArcitcDEM:
+
+* `arcticdem_v4.1_32m_greenland_dem_only.zip`: A ~4.1GB package containing only
+  the 32-m resolution DEM.
+
+* `arcticdem_v4.1_32m_greenland_dem_full.zip`: A ~9.5GB package containing the
+  32-m resolution DEM and ancillary datasets (e.g., a "count" dataset giving the
+  number of contributing DEMs for each pixel).
+
+Download one of these add-on packages and place it in a suitable location in your
+filesystem. Unzip the package and then follow the directions in the `README.txt`
+file included.