clarify 2 tif versions

README.md

@@ -9,8 +9,9 @@ OPHZ currently covers the 48 contiguous US states (US48), and is available in th
 * **shp** - national polygon shapefile
 * **geojson** - polygon geojson files (one for each state)
 * **topojson** - national topojson file
-* **tif/ophz-us48.tif** - GeoTIFF raster.  Each colored pixel of the original map image has been translated to a value representing lower end of the corresponding zone's temperature range.  Most gaps have been filled with the nearest data value.
-* **tif/ophz-alpha-us48.tif** - For those who want the original, unfilled raster with no interpolation of nodata values.  International borders, coastlines, and a few major rivers appear as -98.  Open water is -99.
+* **tif** - GeoTiff raster, two versions:
+  * ophz-us48.tif - Each colored pixel of the original map image has been translated to a value representing lower end of the corresponding zone's temperature range.  Most gaps have been filled with the nearest data value.
+  * ophz-alpha-us48.tif - For those who want the original, unfilled raster with no interpolation of nodata values.  International borders, coastlines, and a few major rivers appear as -98.  Open water is -99.
 
 All vector formats are in WGS84 (EPSG:4326), and the raster formats are in the original Albers projection (EPSG:5070).