Clarify that CoverageJSON is currently limited to 4 spatio-temporal dimensions

[chris-little]

The abstract definition of a coverage allows Domains to multi-dimensional: 1,2,3,4 or more. I think we should clarify the current CoverageJSON restriction to x,y,z,t. This prepares the ground for loosening the restriction.

[jonblower]

CoverageJSON allows for any number of dimensions. It's only the domain types that restrict to xyzt. So I don't think this issue is needed

[chris-little]

@jonblower I corrected the English text to reflect the Domain Type restriction. Is the text in the PR #221 helpful? I think so.