supplement

textual matter that is added onto a publication; usually at the end

a quantity added (e.g. to make up for a deficiency)

a supplementary component that improves capability

add as a supplement to what seems insufficient; "supplement your diet"

serve as a supplement to; "Vitamins supplemented his meager diet"

add to the very end; "He appended a glossary to his novel where he used an invented language"