[doc] add get it started doc.
[doc] add get it started doc.

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