Silent noop on put_credentials in Authentication example

[thomaschandler]

In the README, the authentication examples make a call to Coherence.CredentialStore.Server.put_credentials/2.

Due to a pattern match on the id key in the user_data map, the examples in the README don't work, returning :ok.

I wonder if it would be helpful to have a different return code if the put_credentials/2 call is a noop, what do you think?

For now, I'm going to submit an MR to update the README so that the example works correctly.

Thanks for creating this nice system!