Add documentation for the .with expectation modifier

Awesome library! This, along with [vagrant-berkshelf](/berkshelf/vagrant-berkshelf), has made Chef development much less painful for me.

After reading #21, it was unclear to me that one could test the attributes with which a resource action is performed simply by adding a `with(ATTRIBUTES_HASH)` to the expectation. Do you think this is worth adding to the README? If so, should I also update the HTML in the `gh-pages` branch, or does that happen automatically?