Document the signature of `swagger_controller`

[Wai Lee Chin Feman]

From reading the examples in the readme, I can't figure out the meaning of the first argument passed to swagger_controller. This first argument is a :symbol, whose name is similar to, but not always the same as, the name of the controller being documented. Beyond that, I'm not sure what it does.

I've experimented with passing in an arbitrary value, such as :zzzzzzzzzz, but thereafter, the string "zzzzzzzzzz" never appears in the generated documentation. Maybe my app is too simple, but it seems like the argument is being ignored?

Also, from a cursory glance at the source code, I haven't found an obvious instance where the value is being used.

Whatever the purpose of this argument is, it would help to document it in the readme.