Add multiline method definition guidelines; fixes #241

Here's the addition:

> Align the parameters of a method definition if they span more than one line.
> 
> ```Ruby
> # starting point (line is too long)
> def some_unfortunate_method(with, an, inordinately, long, list, of, required, arguments, that, also, has, options = {})
> end
> ```
> 
> ```Ruby
> # bad - single indent
> def some_unfortunate_method(with, an, inordinately, long, list, of, required,
>   arguments, that, also, has, options = {})
> end
> ```
> 
> ```Ruby
> # good
> def some_unfortunate_method(with, an, inordinately, long, list, of, required,
>                             arguments, that, also, has, options = {})
> end
> ```