If you haven't yet, I would write a letter to her teacher, the principal, and the school counselor that details her diagnosis and your hopes/goals for her this year. I would also look into services for her if you think that would be beneficial (an IEP).
I completely agree that dietary changes that eliminate food intolerances of allergies can alleviate ADHD symptoms dramatically.
You can try an elimination diet or blood test for food allergies, but removing artificial colors*, preservatives, and generally all junk food is a good place to start.
TV and other "screen time" (iPAD, computer) can also trigger ADHD symptoms, so removing that element completely could offer major benefit as well.
I will also add that just this week I am removing my daughter from Kindergarten to homeschool. That's likely not an option for you, but I wanted to tell you that she has had emotional issues we have never seen before now that she has begun kindergarten because of the rigorous schedule and environment, and we are pulling her out for her wellbeing. She was also in preschool for two years and did wonderfully the entire time. I've heard from so many other parents that "kindergarten is just a difficult year" but I wonder why no one has questioned why it is so difficult. I believe 5-years-old is too young for the scheduling and rule-following expected in kindergarten today, and that's a big reason "why".
Good luck to you and your daughter!
(*banned in the UK for causing hyperactivity and found in most kids candy, breakfast cereals, etc. in the US)