Update README

The README should have links to the latest source release and PyPi.
An alternative implementation (untested):

    with sf.SoundFile('myfile.wav', 'rw') as f:
        pos = 0
        while pos < len(f):
            f.seek(pos)
            data = f.read(1024)
            f.seek(pos)
            f.write(data*2)
            pos += 1024