There's so many different identifies and experiences covered by non-binary gender that it's very difficult to make specific recommendations but that's not a reason not to mention them! By including even a short mention of non-binary people in a sports policy you send a message that they are welcome and can talk to the sports body about any concerns they might have.
In deciding which categories a person would be able to compete in, comparison can be made to hormonal statuses of trans women and men. If they wish to compete in a category they are not considered eligible for then arrangements should be made to compete as a guest- reflecting those that would be made for a trans woman wishing to compete in the woman's category before eligibility. It may be that the person wants to be a guest without a gender category, given the lack of options other than male or female, in which case they should be able to compete as a guest outwith of either gender category.
In deciding which categories a person would be able to compete in, comparison can be made to hormonal statuses of trans women and men. If they wish to compete in a category they are not considered eligible for then arrangements should be made to compete as a guest- reflecting those that would be made for a trans woman wishing to compete in the woman's category before eligibility. It may be that the person wants to be a guest without a gender category, given the lack of options other than male or female, in which case they should be able to compete as a guest outwith of either gender category.