This choice quote comes from Kotaku:
Alan and I have always resisted doing any sort of back-story to the Watchmen graphic novel — at various times it’s been suggested that we could do the Comedian’s Vietnam War Diaries or Rorshach’s journal, which we thought would be a bit dopey. But the precedent is, at the time the original comics came out, Mayfair games did a role-playing game that Alan helped write bits of, and it’s completely canon, so this game uses a lot of that less-well known material.
The story they got it from is here. Also of interest to comics fans: Len Wein, the book’s editor, is writing the game’s script. So that could be interesting, although he’s far from the first comics writer to do a script for a superhero video game.