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.
So, now, of course, running `rustdoc` on `lib.rs` does work. I have re-compiled rustc between then and now, and I basically assumed that was probably the problem.

This does not render correctly, but adding a `//!` in doesn't seem to fix it either.