Add "How does Workless work?" to the README

Hi,

We just discovered workless, and it is pretty awesome.
Saves us quite a lot of money that we would've spent on idling Heroku workers.

The thing is, I had to clone it and read the source just to figure out how the workers were started.
It seemed like too much *magic* to allow without understanding how it worked behind the scenes.

I added a few lines to the README explaining this.

--
Timothy Andrew
[nilenso.com](http://nilenso.com)