Add “Custom block helpers” section in README.md

After reading [this article](http://www.saturnflyer.com/blog/jim/2013/10/21/how-to-make-your-code-imply-responsibilities/), we had the idea to integrate a simple DSL to create block helpers for presenters.

This is how we would use it.

@simonprev @garno If you guys like it, I will write the code for it :smile: