contract/contract_variable_quantity/readme/USAGE.md

To use this module, you need to:

1. Go to Invoicing > Customers > Customers Contracts and select or create a new contract.
2. Check Generate recurring invoices automatically.
3. Add a new recurring invoicing line.
4. Select "Variable quantity" in column "Qty. type".
5. Select one of the possible formulas to use (previously created).