Add TreeSet example

Someone asked for an example usage of this on IRC, so I tossed together the simplest one. Obviously, this isn't up to snuff, but it's better than nothing.
We haven't really figured out the best way to do this yet, for example the [hashmap](http://doc.rust-lang.org/std/collections/hashmap/) type has a small module description but a large [type description](http://doc.rust-lang.org/std/collections/hashmap/struct.HashMap.html).

On the other hand, the [result module](http://doc.rust-lang.org/std/result/index.html) has a large description while [the type](http://doc.rust-lang.org/std/result/type.Result.html) has a small description.

I suppose as long as it's somewhere it's ok. I think for collections they're susceptible to being reexported a good bit, so maybe reexported things favor the type while other things (like result) are up in the air?