Currently, Ready or Not's controller support is not official. The game's developers (Void Interactive) have said that they intend to fully support popular controllers. However, until then, you can use some unofficial methods (such as going through Steam's controller support; details in the linked guide). Aiming with controllers is slightly harder than with a mouse and isn't recommended for competitive gameplay. Yet because Ready or Not is still only cooperative, there should be not large loss to using a controller with your pistol loadout in Ready or Not.