Add a note about running time, see #72

doc/0.getting_started.md

@@ -43,6 +43,10 @@ The available commands are:
 * `serve` to serve the built-in webapp with the development server. Do not use
   in production.
 
+_Note:_ Fetching flats can be quite long and take up to a few minutes. This
+should be better optimized. To get a verbose output and have an hint about the
+progress, use the `-v` argument.
+
 
 ### Common arguments