Add broth, shallot and vinegar to the pan and increase heat to high. Cook, scraping up any browned bits, until the liquid is reduced by half, 2 to 3 minutes. Add rhubarb and honey; cook until the rhubarb is tender-crisp, about 1 minute. Stir in tarragon, the remaining 1/4 teaspoon salt and any accumulated juices from the chicken. Serve the chicken with the sauce.