Enhancement: Document setAttributes

[SilverPaladin]

Had to read through code to figure out how to add an attribute to the generated <ul> element.

To add a class to the containing <ul> do this: $menu = new Tlr\Menu\MenuItem; $menu->setAttributes(['class'=>'nav classes']);

[tedslittlerobot]

Good point - when writing the readme, i was more focussed on getting a bit in about all the possible things. I might take an opportunity to write a slightly more fully featured one...!