Mention the newly added Moose::Util::is_role() in Moose::Role

Moose::Role is a likely spot to look for such a function.  Noting the
metaclass involved may be useful for folks rolling their own is_role()
checks.
Merge pull request #61 from tsibley/mention-is_role

Mention the newly added Moose::Util::is_role() in Moose::Role