Readme update

[pjaol]

This did the trick - thank you for releasing it!

There's one update I'd suggest to the readme
When creating a user pool client in the old user pool, untick "generate secret" it's enabled by default; otherwise you'll get an error

authenticateUser: error {
     "message":"Client XXXXXX is configured for secret but secret was not received",
      "code":"NotAuthorizedException"
.........
}