Book: Section on config-description #167

As requested in #166 (comment) I'd like to open up a discussion about config description.

The general idea of what I call "config description" here is that a given configuration type can describe itself in a machine-readable way, which can ultimatively be rendered to user-documentation.

A user would ask the program "tell me how I can configure you?" and the program is then able to tell the user "I get configured via the following settings: 'greeting_message', which has to be a String, 'repeat', which has to be a number greater than zero".

A possible solution for this is sketched out in #166 using the type_description crate.