README 'quick start' update

Right now, testing with Avail GTFS feed and 'wget' directly
to protobuf file that is interpreted.
I think occasionally the 'wget' half finishes when trying to load
the file, leading to corrupted data.

Instead, 'wget' to temporary file, then 'mv' over in the hopes
the move is atomic.

experimental/README.md

@@ -19,7 +19,12 @@ To get a rudimentary real time bus map, execute the following:
 
 ```
 cd ../html/
-while [ true ] ; do wget -q 'https://realtimetcatbus.availtec.com/InfoPoint/GTFS-Realtime.ashx?&Type=VehiclePosition&serverid=0' -O VehiclePosition.pb ; echo '...' ; sleep 10 ; done
+while [ true ] ; do
+  wget -q 'https://realtimetcatbus.availtec.com/InfoPoint/GTFS-Realtime.ashx?&Type=VehiclePosition&serverid=0' -O VehiclePosition.pb.tmp ;
+  mv VehiclePosition.pb.tmp  VehiclePosition.pb  ;
+  echo '...' ;
+  sleep 10 ;
+done
 ```
 
 ```