[doc] add note about forcePrepend arg of addApi method to UPGRADE.md
[doc] add note about forcePrepend arg of addApi method to UPGRADE.md
digisec/Payum,formapro-forks/Payum,fullpipe/Payum,staabm/Payum,Payum/Payum,digisec/Payum,StoreFactory/Payum,StoreFactory/Payum,StoreFactory/Payum,ASKozienko/Payum,formapro-forks/Payum,aitboudad/Payum,Payum/Payum,fullpipe/Payum