Add documentation for the geonames API that provides ground altitude

README.md

@@ -47,6 +47,12 @@ the sea level, so you need to figure out the ground altitude and add the height
 of the reference point (e.g. height of the building).
 In the future, the ground altitude will be computed automatically.
 
+In the meantime, you can use the geonames API to get the ground altitude:
+
+    lat=42.4
+    lon=0.54
+    curl "http://api.geonames.org/astergdem?lat=$lat&lng=$lon&username=celutz&style=full"
+
 *Hint: it is better to set the altitude of the topmost part of a building.
  This is because it will be much easier to aim when viewing a panorama.*