Add have(:no) to README

[vassilevsky]

Hello :)

I was searching today for a natural way to express the expectation of an empty Rails association.

First I wrote this, hoping it would work:

expect(bird).not_to have.feathers

This did not work :)

I dived into the source and discovered that I could do this:

expect(bird).to have(:no).feathers

This is also a good way to make the expectation sound like natural English :)

So I thought that it's worth mentioning in the README.

[JonRowe]

Thanks, an excellent documentation contribution 👍

[pirj]

Thanks! Didn't know this was possible 👍