8.0 add account cash discount compute ape

cc/ @adrienpeiffer 

PR to support internal review. Do not merge.
Add a README.rst for the description.

# Belgium cash discount computation

The module implements the Belgian method of applying taxes in presence of cash discount.

The cash discount percentage is applied is applied on the amount without tax, and the VAT is computed on the discounted amount.

## Credits

* the computation algorithm has been designed by Luc De Meyer (Noviat)
* ACSONE implemented the current module structure and integrated with OCA/bank-payment