You can simply highlight this in the free format narrative of your listing when you describe the main features of what you provide. There is also a tickbox in the Amenities section to show that breakfast is provided, though prospective guests may not notice it to that level of detail when they read your listing.
As this is a significant added value item which guests should appreciate you could also highlight it in your introductory correspondence with new guests, and stress that there is no additional charge for breakfast. Hopefully this may help with their value perceptions of their stay, and eventually on the ratings that they will award you.
