@Matthew346There is no provision for adding an additional pet waiver through Airbnb. You should set up your pet policy as part of your house rules: that's the only way to do it through the Airbnb system. You could ask your guests to review something outside of Airbnb on check in, but you should also make sure the same information is in your rules so people can't claim they didn't know what they were getting into.
I do allow pets in my primary listing, and I just have my policies laid out in my house rules. So far, so good. I personally don't have specific rules around dogs (which are the only animals I allow), but I've seen a lot of listings that limit weight (say only pets under 35 lbs, if pets are left alone in the space they need to be crated, and no pets allowed on the furniture). I would also mention picking up after pets and a mention of leash laws, if applicable in your area.
Also maybe think about adding a pet fee for extra cleaning. If you want to add one, just mention what you charge in your listing, then send a request for money when the guest books.