docs: function verifyRequestSignature(req, res, buf) {
      var signature = req.headers["x-hub-signature"];
  
      if (!signature) {
          // For testing, let's log an error. In production, you should throw an 
          // error.
          console.error("Couldn't validate the signature.");
      } else {
          var elements = signature.split('=');
          var method = elements[0];
          var signatureHash = elements[1];
  
          var expectedHash = crypto.createHmac('sha1', APP_SECRET)
              .update(buf)
              .digest('hex');
  
          if (signatureHash != expectedHash) {
              throw new Error("Couldn't validate the request signature.");
          }
      }
  }