Add international_development_fund schema

This format will be published by DFID editors via Specialist Publisher.

See also: https://github.com/alphagov/finder-api/pull/33
This file is UTF8 encoded, which should be fine since that's always the character set of JSON (according to the JSON spec, anyway, as if anyone follows specs).  https://github.com/alphagov/rummager/pull/275 tries to make our code read the file as UTF8.