Goodies: Adding documentation for roles & helpers

Related to/depends on: 
* https://github.com/duckduckgo/zeroclickinfo-goodies/pull/576
* https://github.com/duckduckgo/duckduckgo/pull/91
"can be used by using" => "can be used by adding the following line, `with DDG::GoodieRole::RoleName`, to your Goodie's Perl module (.pm file).

Would like to make it a little more clear how exactly this get's used.