sell_stale.json.example is incomplete

[skoenig]

If I rename and use the default sell_stale.json.example config file, I encounter this exception:

./src/investhor/scripts/sell.py 
Traceback (most recent call last):
  File "./src/investhor/scripts/sell.py", line 108, in <module>
    main()
  File "./src/investhor/scripts/sell.py", line 89, in main
    discount = params[discount_name]
KeyError: 'no_discount'

Seems like the example config file is missing some parameters and they are also not documented.