Add examples using should and expect in README

It's a bit silly, but when I tried to use `sinon-chai` I was using the `should` syntax from the README (I hadn't noticed the bit "the `expect` equivalent is also available" under "Assertions"), and was getting an error because my tests are using `expect` so I hadn't called `chai.should()`.

Let me know what you think, but I tried to make it more explicit that you can use both `should` or `expect`, and also added full examples (that you could use to check you've got everything set up and working properly, in my case I would've seen the missing `chai.should()` statement).