Add information about the REPL and hot code reloading

I kept referring to [this](https://groups.google.com/forum/#!msg/riemann-users/JQg3TIbbqXA/E3mdqPyjUQAJ) mailing list post about using the REPL. So I thought it might be useful to have it in the docs.

I've also added a quick note about hot code reloading since thats what sparked off the original thread.