I would ignore advice on commenting. Things should be clear, not commented. Docstrings are nice if you are going to generate some documentation for end users; googling for "string isempty java" is easier than opening up java/lang/String.java and searching for "empty" in your text editor. (But both techniques have their advantages and disadvantages.)
Tests are a great way to show what your library can do. I recommend adding those before you worry about documentation. (How do you know your code, as posted, works?)
One thing I have to decide on this project is whether I'm going to follow the convention (which I don't like) of putting "test/" in a separate directory from "main/". I dislike it because I think unit tests should be included inline if short and relevant to the purpose of the function.
Generally, my testing practice is to REPL-test and then include the tests in code for posterity. It makes the testing process more playful to start in REPL testing. I know it's the opposite of TDD but it works for me.
Michael, could you explain why you are using Scala over Clojure? I know you've got experience with both, but Clojure has always seemed more appealing to me.
Tests are a great way to show what your library can do. I recommend adding those before you worry about documentation. (How do you know your code, as posted, works?)