docs: function parseReadme(book) {
      var logger = book.getLogger();
  
      return parseStructureFile(book, 'readme')
      .spread(function(file, result) {
          if (!file) {
              throw new error.FileNotFoundError({ filename: 'README' });
          }
  
          logger.debug.ln('readme found at', file.getPath());
  
          var readme = Readme.create(file, result);
          return book.set('readme', readme);
      });
  }