Perhaps is it easier to implement than pop-ups but you have to access the great library first.. which means you have to start a game and that means you already have chosen the options. If creating pop-ups asks too much work, the easiest thing to do is to write a README text file. But even if there is a great library entry or a text file, pop-ups are good reminders, you are right Maq.
Popups might work, but you could add a description screen on the right side of the options. Opening the Great Library is too much and for the readme, well who reads readmes.