[#64] Showcase specification

Please use this Pull Request for discussion related to the new CKAN 'Showcase' feature (aka, Related Items). **DO NOT MERGE** until the spec has settled (likely when the feature is released).
> As I see it, there is a problem with using Apps in that many people will associate it with the narrow category of type meaning 'stand-alone apps, probably mobile', rather than the broader, 'application of use'. We don't want to restrict reuse examples to just the former meaning.

+1