[omnipay] add note to upgrade file
[omnipay] add note to upgrade file

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