Added RDoc to Leaderboard class initializer

I was having issues using your API due to lack of documentation. So after feeling that I understood it a bit better from going through the code, I thought I'd contribute back some basic RDoc for future users.

If this sort of thing is welcome, I'd be happy to add more RDoc for other commonly used methods.

Let me know if there's any documentation format issues, or standards that you want to be followed for doc contributions.

(Apart from documentation, your API has been wonderful! I am using it to gather data for a small research project I am currently working on. Thanks for providing it open source!)