New rustpkg tutorial.
Updating rustpkg tutorial from feedback.
auto merge of #9239 : steveklabnik/rust/rustpkg_tutorial, r=catamorphism

First shot at a new tutorial for rustpkg. /cc @catamorphism

Right now, I'm linking to my sample package on GitHub, I'm not sure that everyone would be comfortable with me having that there. Maybe under the mozilla org? I think having one to install and hold up as a default makes sense.