The validate_token function in the readme is missing a parameter

[Carrigan]

Thanks for the great gem! I was setting this up and testing the endpoints using the readme documentation and kept having the /validate_token endpoint fail. After searching around a bit, I found that this endpoint requires three things: the uid and access-token as listed in the readme, but also the client header given in the sign-in request. Once client was added, everything worked just fine.

I will follow this with a PR shortly to correct this documentation as well as noting that the client and access-token are given back in the headers of the /sign_in response since this was not immediately obvious. Hope that this helps!