Add additional_fields to schema

[IanLee1521]

Add an additional_fields field to the release object in the schema. This would allow individual agencies to add additional information to their releases.

[emartinez-usgs]

It looks like in most places the schema specifically declares additionalProperties: false, but on the release object, no such flag is specified. The default behavior then is any arbitrary properties may be added to an item in the releases array and that is still valid per the schema.

Can you clarify your request @IanLee1521 ?