[paypal-pro] add notes about chages in paypal pro to upgrade.md.
[paypal-pro] add notes about chages in paypal pro to upgrade.md.

Payum/Payum,formapro-forks/Payum,StoreFactory/Payum,StoreFactory/Payum,StoreFactory/Payum,Payum/Payum,aitboudad/Payum,fullpipe/Payum,formapro-forks/Payum,fullpipe/Payum,staabm/Payum,ASKozienko/Payum