Add something about PlayerId getting added to Handle(Login|Register)Resp.

[ajmcmiddlin]

In Exercise 4 it seems that the HandleLoginResp and HandleRegisterResp constructors both get a PlayerId value added to them. I'm assuming this is to preserve the player ID that was given at submit time, as opposed to whatever happens to be in the field when the backend responds. In any event, I don't see this mentioned in the instructions anywhere.