That's by design.
The .txt it opens is the README, which tells you how to use the cancel mechanic to get past the age gate. The mechanic used to get past the age gate is used in almost every choice presented to you, and is required to get past one other part in the game that people tend to get tripped up on because they forget about that mechanic.
There's also some other useful info in there besides the usual changelog/contact/generic README stuff that I'd recommend giving a look, including info on how to transfer a save from the short prequel, Arrival.