What is a Module Attribute and How do I define/use it?

The Elixir book uses module attributes such as `@moduledoc` and `@create_attrs` in modules and tests but does not _explain_ what they are and how to use them, we should add an explanation to our tutorial...

http://brettwise.com/the-or-at-symbol-in-elixir/
https://elixir-lang.org/getting-started/module-attributes.html