Feature/resource descriptor documentation

@pservedio and @chaimsolomon Please review and merge.

This PR documents the main concepts associated with the various sections of a _Resource Descriptor_. 

Note: the large @title tag is used by YARD to create nicer file names/document URIs. However, since it must be in a comment #, it looks like a markdown heading here. The documentation link on the README page now links to a current rubydoc repo for the docs.