Skadi is weird magic

Says Nathaniel

Author: Katie Kowalsky

docs/use-service.md

@@ -27,6 +27,8 @@ You can request tiles using Mapzen's global CDN:
 ##### Skadi
 
   `https://tile.mapzen.com/mapzen/terrain/v1/skadi/{N|S}{y}/{N|S}{y}{E|W}{x}.hgt.gz`
+  
+  Note: Skadi files are split into 1° by 1° grids. File names refer to the latitude and longitude of the lower left corner of the tile - e.g. N37W105 has its lower left corner at 37 degrees north latitude and 105 degrees west longitude. For example:  N37W105: `https://tile.mapzen.com/mapzen/terrain/v1/skadi/N37/N37W105.hgt.gz`.
 
 #### Additional Amazon S3 Endpoints