Update README

The README should have links to the latest source release and PyPi.
OK, I am going to disagree with this. In practice, the garbage collector *does* work. If you create a SoundFile object in some function, the file *will* be closed as soon as the function returns. There are ways to make sure that this happens at a predefined point in time, but they are *not* necessary in most cases, especially not for interactive work.

I think that this is actually an important feature, and should be advertised as such. I will change the specific wording, but the feature itself is useful and should be advertised.