code: func saveCertResource(storage Storage, cert acme.CertificateResource) error {
  	// Save cert, private key, and metadata
  	siteData := &SiteData{
  		Cert: cert.Certificate,
  		Key:  cert.PrivateKey,
  	}
  	var err error
  	siteData.Meta, err = json.MarshalIndent(&cert, "", "\t")
  	if err == nil {
  		err = storage.StoreSite(cert.Domain, siteData)
  	}
  	return err
  }
docs: // saveCertResource saves the certificate resource to disk. This
  // includes the certificate file itself, the private key, and the
  // metadata file.