Update README.md

[vit100]

Added reference to jwks-rsa project with sample code

[jfromaniello]

Unfortunately this will no longer work with v7 as I changed the signature of the secret function:

https://github.com/auth0/node-jwks-rsa/blob/master/src/integrations/express.js#L25-L36

We will need to change this to something like this:

  return async function secretProvider(req, token) {
    if (!token.header.alg || !supportedAlg.includes(token.header.alg)) {
      return;
    }
    const key = await client.getSigningKey(token.header.kid);
    return key.publicKey || key.rsaPublicKey;
  };