Update README to mention uniqueness handling

[jklausa]

I know it's documented in the DOCC-generated pages, but when I was first looking into Boutique, it took me a long time to find whether this was handled automatically, or if I had to roll my own solution.

Feel free to rewrite this example or mention it in some other way, but I feel that this aspect of Boutique deserved to be highlighted in the "main" README upfront.

[mergesort]

This is a really good idea, thanks for adding it @jklausa! I tweaked the wording a tiny bit, but I appreciate the suggestion and will merge in your change. 🙂

[jklausa]

🫡