You will need the pickaxe for this, so play the story tell you get it.

There is a stair case near were you begin. Go to the top of it.

The wall right in front of the top of the stairs is were the room hides, if you walk up to it you will see the prompt to clear debris.

Once you clear it there will be a table inside with some blue prints.

Rejoice for you have found the secret room!