Remove spec and metadata notes from the README

The spec items have since been reflected in the issue queue
at https://github.com/openelections/openelections.github.io/issues/

The notes about the metadata API are less relevant since we've
decided to store metadata in static files.  Also, there's less of
a need for those notes now that more of the site has been
implemented.
Add documentation for updating metadata JSON

The metadata JSON files exist in this repo, but are generated by
commands that live in other parts of OpenElections' infrastructure.

Add documentation for how to regenerate the JSON files to this
project's README for easy future reference.