Update README

The README should have links to the latest source release and PyPi.
"documentation" is an old branch. The pages on readthedocs were still available, but after a reload, they are not anymore. For now, there is only "latest", but as soon as 0.6.0 is out, it will also be available there.

This should be the correct address: http://pysoundfile.readthedocs.org/, no need to give a separate link name.

I personally prefer the short form http://pysoundfile.rtfd.org/, but that's of course a matter of taste.

In any case, we should use the same link everywhere (currently in `pysoundfile.py` I used the short version).