Odd. Is there an info text about that in the english version? Since i cant find any info in my version.
Nope, just the same info in English i think. (Construct a deck using only cards from Whispers of the Old Gods, Mean Streets of Gadgetzan and Rastakhan's Rumble)
Try with DH and add only 1 card for example. You will start the game with like half a deck.
And protip if you're struggling to win a match: Choose warlock and add only your C'thun cards to your deck. You'll get some random cards but your deck will probably be less than full. That way you can draw your C'thun easier. The warlock hero power also helps there