GetAuthnRequestURL has new, undocumented parameter

[mfitton]

It appears that there's a new variable in GetAuthnRequestURL called state which refers to the Relay State of the request. However, in the usage documentation in the README, the variable is not accounted for. I would add the usage myself, but I'm unsure of the purpose of the parameter.

[antoni]

I have added relevant information to README (see #29). That parameter depends on the actual service provider's logic.