The most common things I hear of people adding to the poaching liquid is vinegar, salt, or baking soda. Vinegar does help set the albumen more quickly, which helps prevent feathering, but people with a keen sense of smell can taste the vinegar in the poached egg, which is why I don't recommend adding it. Salt can be added if you want as it will help season the egg, but it has no impact on how quickly the egg white sets and is not necessary. As for baking soda, I'm not sure where this idea comes from, but it has no impact on setting the egg, and it gives the eggs a metallic taste, so don't add it.