Update README to current state of code

example/README.rst

@@ -12,8 +12,11 @@ SST is a simple one that uses the default csppfetch.Downloader.
 
 GFS ancillary data needs to be retrieved in pairs; only half of a pair is not
 useful.  Also, while new data is available every 6 hours, which data you use
-changes every 3 hours.  To handle this, the GFS Downloader inherits from
-csppfetch.Downloader and customizes some behavior.
+changes every 3 hours.  To handle this, the GFS Downloader uses
+csppfetch.BoundedForecastDownloader.
+
+RAP data covers three areas. A specialization of csppfetch.Downloader is used
+for common elements, then three instances are created for the three areas.
 
 aitf-update-cache
 -----------------