Add threading macros and simple transducer examples

[the-frey]

This is a small PR to add threading forms (-> and ->>) as they are ubiquitous within Clojure codebases and worth introducing to newcomers early (whether that should be bundled in with the list ops is another question outside of this PR).

I also added a couple of tiny transducer examples so that they're represented as well.

[the-frey]

Thanks for the feedback @trptcolin - will update and get it sorted asap.

[the-frey]

Okay, I've changed that up a little. Thoughts? Still unsure if the transducers example is too in-at-the-deep-end...

[trptcolin]

Great - thanks! 🎉