Add "NOT IMPLEMENTED" to show.receiver.list_all

In fact, the command is correctly implemented
but the receiver table is always empty.

ogn/commands/showreceiver.py

@@ -13,7 +13,7 @@ receiver_beacons_per_day = 24 * 60 / 5
 
 @manager.command
 def list_all():
-    """Show a list of all receivers."""
+    """Show a list of all receivers (NOT IMPLEMENTED)."""
 
     timestamp_24h_ago = datetime.utcnow() - timedelta(days=1)