Showcase specification

This Pull Request refers to issue #64.

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).
> Another consideration is that no matter what word you use, just a button with that word may not intuitively encourage people to report reuse

and

> It sounds like a CKAN page needs some text to the effect of: "Have you made something with this data?...

Agreed - several instances from the survey made a direct appeal for submissions in a fairly prominent way.