Remove the blocking loop-back example from the README file

[mgeier]

The blocking read() and write() methods shouldn't be used together.
If synchronized input and output are needed, the callback API should be used.

The blocking methods may work in some cases, but they don't work in other cases, see for example #25.

[bastibe]

Actually, if I understand the Portaudio documentation correctly, they should work together. I have seen a few contexts in which they don't. It seems that Portaudio runs into timing issues if you try to read and write simultaneously from a stream that uses different physical audio devices for reading and writing.

We should definitely investigate this further. Maybe there is a way for a guaranteed-working readwrite method or something like that.