[FIX] missing documentation for field payment_type

Created by: zaoral

module: payment_term_type

The Payment Type field in the payment.term model NEED TO HAVE some help: There is not help related.

This the field definition that need to be updated indicating when the field is set as credit and when the field is set as cash.

    payment_type = fields.Selection(
        [('credit', _('Credit')),
         ('cash', _('Cash'))],
        string="Payment Type", compute='_compute_payment_type')

original code here

Also the module will be very helpfull if have some README.rst information about how does the account.config.settings wizard bdp/bqd configuration affects to this module