[doc] add installation part to get it started.
[doc] add installation part to get it started.

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