Clarify frequency 0 documentation

@samcday Tremendously useful library you have here. And kudos to @Sunib for the new frequency 0 feature added in #7. In fact, I always assumed that frequency 0 was already supported. :flushed: 

I added some clarification to the readme that may be helpful. I had some unit tests that set frequency to 0 I had to fix when upgrading to the latest version of stream-buffers. (Really easy -- in most cases I piped the readable stream buffer to a destination stream prior to inserting data into the buffer.) 

Lastly, as a future enhancement, when frequency is set to 0 I suggest starting the readable stream buffer  paused. It's standard streams2 behavior as node 0.10 to wait until consumers are ready before emitting data.