Add example for content_first

[KodrAus]

Add an example that demonstrates the content_first method. Maybe this can just be another example in the crate root docs. With a comment like:

// When contents_first is true outputs:
// `dir/first_file`
// `dir/last_file`
// `dir`

Does anyone have any thoughts on how the docs could illustrate the results of certain filters better?

[steveklabnik]

Does anyone have any thoughts on how the docs could illustrate the results of certain filters better?

One way would be to show results both with and without the filter; I think this is what you're alluding to above but am not sure.

[KodrAus]

@steveklabnik Yeh I was thinking something like that, maybe a hypothetical directory in a list or a tree-like ASCII diagram.