There is currently no specific documentation for integration for using PI to the ECC background. This would be done using the available web services from Ariba. Please review the Ariba integration documentation to learn about the available web services. We do have on our roadmap to add out of the box integration between Ariba Contracts and SAP ECC. We are looking at adding this with our 15s Sourcing release due out in late 2015 early 2016.