Update README

The README should have links to the latest source release and PyPi.
> Should we really care?

Yes.
Python doesn't have constructors and destructors like C++. We should try not to spread such mis-information.

The mere existence of the `__del__()` method might suggest something like that, but it's not our fault that Python has a `__del__()` method but doesn't force its invocation.

I think it is *very important* that we tell our users the truth about all this.

> The SoundFile will be destroyed eventually.

Probably. I guess the OS will close the file when the interpreter is closed. Probably it's closed earlier. Probably not. Who knows.

Anyway, it's not guaranteed by Python, so we shouldn't guarantee it either.

> I don't think that the distinction between out-of-scope and collection time is significant in practice.

I think it may be in an interactive sessions. There might still be some reference to the file lying around and the session might be kept open for a long time.

But that isn't even the point. The point is, that garbage-collection isn't guaranteed at all!

Therefore, we should advertise the two (and only two) Pythonic ways (ideally in this order of preference):

1. use a context manager
2. call the `close()` method
3. don't care (but don't rely on `__del__()` being called)