Add pointer regarding unit tests as inner classes

@benjchristensen - If I hadn't stumbled over your respective [pointer](https://github.com/Netflix/RxJava/issues/154#issuecomment-14544894) by chance, I would have asked the same question as Matt Bishop when encountering this README.

Your reasoning for embedding unit tests as inner classes is inspiring (thanks much) and a helpful explanation for those who disagree regardless, so I suggest to promote it accordingly.