code: private static Properties mergeExtraHeaders(File baseDir, Properties properties) throws IOException {
          File extra = new File(baseDir, EXTRA_HEADERS_FILE);
          return Instructions.merge(properties, extra);
      }
docs: If a bundle has added extra headers, they are added to the bundle manifest.
  
  @param baseDir    the project directory
  @param properties the current set of properties in which the read metadata are written
  @return the merged set of properties