Clarify "Script App" should be selected when registering the app

[jakemalachowski]

I was getting 401s when getting started and it took me some time to notice that I should have selected Script rather than Web App when registering my app. This is shown in the screenshot of the registered app, but not specifically called out so it's easy to miss.

[janitha09]

@turnage where is the source for the gitbook? didn't notice a doc folder in the repository