Nouns ending in -ou, -out, or -oux are usually masculine.
The whole family: -ou, -out, -oux. All masculine.
Is ajout masculine or feminine?
l'ajout
The rule behind it
91.5% of -ou / -out / -oux nouns are masculine
In context
L'ajout de sel améliore considérablement la saveur.
The addition of salt greatly improves the flavor.
C'est un ajout précieux à notre collection.
This is a valuable addition to our collection.
Notez l'ajout au dernier point de la première page.
Notice the addendum to the last item on the first page.