cloud_cover and snow_cover calculation guidelines

README.md

@@ -64,7 +64,9 @@ of the Earth, so the cloud cover value would apply to all assets.
 
 Estimate of snow and ice covered surfaces as a percentage (0-100) of the entire scene. If not available, the field should not be provided. Generally, 
 this value should be used in Item Properties rather than Item Assets, as an Item from an electro-optical source is a single snapshot 
-of the Earth, so the snow cover value would apply to all assets. 
+of the Earth, so the snow cover value would apply to all assets.
+
+It is important to consider only the valid data regions, excluding any "nodata" areas while calculating both `eo:cloud_cover` and `eo:snow_cover`.
 
 ### Band Object