_fetchReadme = async function(pkg, packageName) {
  logger.info({ pkg: packageName }, "_fetchReadme");
  const langs = ["en-US", "zh-CN"];
  for (const lang of langs) {
    const readmePathPropKey = PackageExtra.getPropKeyForLang(
      PackageExtra.propKeys.readme,
      lang
    );
    const readmePath = pkg[readmePathPropKey];
    if (readmePath)
      await _fetchReadmeForLang(pkg, packageName, lang, readmePath);
  }
}
Fetch repository readme.
@param {object} repo