[doc] add paypal pc upgrade notes to main UPGRADE.md file
[doc] add paypal pc upgrade notes to main UPGRADE.md file

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