Mention in README about 100% CPU usage with non-blocking mode.

Hello.

Firstly thanks for making this awesome library. I used PTT to create a CLI Twitter client here:
[https://github.com/DTVD/rainbowstream](https://github.com/DTVD/rainbowstream)

In my development, someone noticed me an issue about 100% CPU usage while using non-blocking mode of PTT. This cause by the iterator yielding immediately every time.

I think it worth to mention in README file.