Hi there,
Your poor chicken is in a little bit of a rough spot! I'm sorry this happened to you and your bird. That certainly doesn't look nice.
First things, I'd cut away feathers from around the wound. They're going to have dirt and dust that carry microbes to get into the wound and infect it.
Second, I'd flush it with sterile saline. If that isn't available soap and warm water works too! I don't think hydrogen peroxide is a good idea in this situation, as it does damage to healthy tissue.
See if you can get some antibiotic ointment on that wound after cleaning and flushing it. Something without an analgesic ingredient (look on the box, make sure it DOESN'T have human pain relievers).
Consider picking up some Vetericyn antibacterial spray or blu-kote from a TSC or feed store near you. The blu-kote will sting and hurt, but I think its antibacterial and antifungal properties and ability to seal the wound are invaluable. If you're up for it, you can inject an antibiotic into her breast tissue or put it in her water to help ward off infection. I mix my own, but you can get an antibiotic suspension from a feed store.
Keep her inside inside a dog crate or plastic tote with soft bedding if you can. Keep her warm. She may not eat or drink for a day, but keep water and proteinaceous foods like boiled egg or canned tuna on offer. Remember that she's in shock and in pain.
To be honest, I am not sure if dressing the wound is going to be appropriate in this situation. Seems like it may do more harm than good, as taking it on and off to inspect the wound may stress the bird. In my experience, chickens can heal relatively quickly. Number one priority is to not let this wound get infected.
