I know there is in-game documentation, but an external wiki would be nice. I am thinking the GitHub wiki would work.

One option even would be to make the guide pull info from this wiki, download it, and use it. any changes to the wiki are altered in the files. Offline? The wiki will just not be updated! Or, it could go more in-depth (After all, you don't need to deal with a menue in the way)

Just an idea

Also, if you just make the first page, I would be willing to start the wiki for you (:

For unknown reasons, I like making wikis...