docs: possibility of using attributes in markup::define!

[j-n-f]

I was working with this crate trying to think of how to log/serialize the objects being rendered into a view (for debugging), and I noticed the code implementing the macro had attributes near where the struct for each template gets created.

I tried it out, and it looks like it's possible to do something like:

markup::define! {
    #[derive(Debug, Serialize)]
    SomeTemplate(a: bool, b: bool, c: bool) {
        // ...
    }
}

It wasn't immediately obvious to me that each entry in define! behaves more or less like struct SomeTemplate {}. I think that's pretty awesome functionality that the docs could highlight.