Document that fetchOne(_:key:) supports multiple-column primary keys
Document that fetchOne(_:key:) supports multiple-column primary keys

zmeyc/GRDB.swift,groue/GRDB.swift,zmeyc/GRDB.swift,zmeyc/GRDB.swift,groue/GRDB.swift,zmeyc/GRDB.swift,groue/GRDB.swift,groue/GRDB.swift