Suggestion: Add basic getting started documentation

Hey, I'm trying out the privacyIDEA PHP-client for the first time.
I would find it helpful if there was some documentation anywhere (maybe it exists, and I can't find it?) which shows how to use the API client.

// Edit: Yes, there are comments in the code which show the right direction, but I think a simple example in the README.md for example wouldn't hurt anybody.