More realistic example into the readme

[allantatter]

The current example how to implement "Authenticating with semantics identifier" (https://github.com/SK-EID/smart-id-php-client#authenticating-with-semantics-identifier) merges 2 steps into a single flow which is a bit misleading when considering a real-world use case that IMHO is split into following steps:

1. First request is made to the backend to initialize the Smart-ID authentication process and get the verification code to show to the user.
2. Right after the verification code response has come back and shown to the user, a second request is made to the backend that responds when the user has gone through the process within Smart-ID app or some exception has been thrown. Then the user is either authenticated or an error will we sent as a response to the client.

It is not so straight-forward to split the example into 2 steps as objects are passed along, not simple data like strings (like authenticationHash, which would happen in a realistic use case when data is moving between the backend and the client between those steps).

I do understand that readme should show a simple use-case however it would make sense in my mind to make it more that can be used in a real application.