code: function parseSummary(book) {
      var readme = book.getReadme();
      var logger = book.getLogger();
      var readmeFile = readme.getFile();
  
      return parseStructureFile(book, 'summary')
      .spread(function(file, result) {
          var summary;
  
          if (!file) {
              logger.warn.ln('no summary file in this book');
              summary = Summary();
          } else {
              logger.debug.ln('summary file found at', file.getPath());
              summary = Summary.createFromParts(file, result.parts);
          }
  
          // Insert readme as first entry if not in SUMMARY.md
          var readmeArticle = summary.getByPath(readmeFile.getPath());
  
          if (readmeFile.exists() && !readmeArticle) {
              summary = SummaryModifier.unshiftArticle(summary, {
                  title: 'Introduction',
                  ref: readmeFile.getPath()
              });
          }
  
          // Set new summary
          return book.setSummary(summary);
      });
  }
docs: Parse summary in a book, the summary can only be parsed
  if the readme as be detected before.
  
  @param {Book} book
  @return {Promise<Book>}