Setup guide need to include password and username when starting hub

[mandelmonkey]

When starting the hub if your counterparty username and password is not rpc:1234 then an error is thrown and need to set manually when starting

picopayments-hub --testnet --host=0.0.0.0 --cp_username=USERNAME --cp_password=PASSWORD --cp_url=http://127.0.0.1:14000/api/