Improve documentation how to use the Book mode

[bernhardreiter]

The book example does not work in v1.0.0.

[s-l-teichmann]

Its works ... I just used it the wrong way. @tschmidtb51

To do it right you have to insert a new type on the top level and reference it later in the document.
E.g to define a mobydick type you have to:

  [types]                                                                             
    [types."mobydick"]                                                                
      type = "book"                                                                   
      path = "mobydick.txt"                                                             
      minlength = 100                                                                 
      maxlength = 1000                                                                
    [types."csaf:#"]                                                                  
      type = "object"   

Later in the template document you can reference it like this

      [[types."csaf:#/properties/document".properties]]                               
        name = "category"                                                             
        required = true                                                               
        type = "mobydick"  

@bernhard-herzog Maybe we should improve the docs a bit?!