Add a note about checking expectations in hooks

[ciarand]

This is the documentation adjustment discussed in #7.

[danielstjules]

I hope I don't sound like a pedant by asking this, but what do you think of making it a smaller example (and using $this rather than the use construct) at the end of the readme under the heading "Notes", or something similar? I'm thinking I'd rather keep the introduction short, and not too full of extra details and highlights of potential issues. Do you agree that those concerns could be left towards the end - for people who are interested in using the lib?

[ciarand]

Not at all. I do think that since they relate primarily to mocking frameworks they should be close to that section - how's this?

[danielstjules]

Good idea, thanks!