Add how to get Topographical Data (#248)

* Add how to get Topographical Data

* Make more readable

Author: jacobbieker

README.md

@@ -102,7 +102,12 @@ TODO
 
 ### Topographical data
 
-TODO
+1. Make an account at the [USGS EarthExplorer](https://earthexplorer.usgs.gov/) website
+2. Create a region of the world to download data for, in our case, the spatial extant of the SEVIRI RSS image
+3. Select the data products you want, in this case SRTM elevation maps
+4. Download all the SRTM files that cover that area
+
+There does not seem to be an automated way to do this selecting and downloading, so this might take awhile.
 
 
 ## Configure `nowcasting_dataset` to point to the downloaded data