Example on how to sign a URL

1. Install dependencies: npm install
2. Copy the file .env.example to .env file and change the secret to a value that only your server will know.
3. Run node sign.js http://localhost:3000/hello (use the url that you'd like to sign)
4. Run the example express application with node index.js
5. Access the signedUrl that was generated on step 3.