Added a section about @task(help=...) in the Getting Started page

I've just learned about pyinvoke's existence today and while playing with it, it took me quite a while to figure out how to add a description to a task argument. Of course, once you know that it's done through a @task arguments, you know that you can look it up in the API section, but when you start using the library, it's not self-evident that an argument description belong in the @task argument. So I poked around a lot to find about that.

I think that for this reason, it needs a mention in the Getting Started page.