Add better expectations examples

[mcbridem]

We should at least document this:

## Canonical check
-   directive: check
    url: https://www.usnews.com
    html:
    -   selector: link[rel="canonical"]
        attribute: href
        equals: https://www.usnews.com

There's no mention of this attribute key in the repository currently, as far as I can tell.

Adding some actual example expectations files might also be nice.

[mcbridem]

Oh, I was looking in my expectations repository, not in this repository. D'oh. :-(

[mcbridem]

I think including an example YAML file, even if it's only testing one URL using lots or all of the available features, would be really helpful.