You could add a script step before your installer step. Save a VBS script with a code like the following example onto your Package share and refer to this script in the first step. It will bring up a popup and ask the user if he would like to continue with the installation.
result = MsgBox ("Do you agree with the installation of Software XXX and reboot?", vbYesNo, "Message Box Title")
Select Case result
Case vbYes
MsgBox("The installation will now begin.")
WScript.Quit(0)
Case vbNo
MsgBox("The installation was cancelled.")
WScript.Quit(-1)
End Select