Add examples for optional names to README

[kenny-evitt]

PR #9 seems to have implemented this feature but there's no other mention of it or any examples.

In case anyone else looks for this using the same search keywords as myself, I tried finding this by searching for "alias" and "rename".

I'll submit a PR if you'd like.

It might also be able to supply optional names with import-vars too, e.g. something like this:

(import-vars
  [clojure.walk
    [prewalk walk-pre]
    [postwalk walk-post]]
  [clojure.data
    [diff data-diff]])

I'll make another issue for this change if you're interested in it (either yourself or by others).