Update README.md

README.md

@@ -56,6 +56,8 @@ _Note:_ You can also write `long` or `longitude` instead of `lon`, or
 
 `description`=True|False works only for ecoregion and landcover
 
+`relative`=True|False works only for topography_srtm
+
 Individual services may have additional optional URL parameters.
 
 ## 2. Individual services