A simple 'Thank you' would do, I guess ;)

Like Damian said, you can just say "thank you." These phrases are normally used to let you know that you can ask the sender a question at any time, even if you don't need help with anything right away. For example, if you realize you have a question three days later, you can still ask the person the question - they will be okay with it. They are polite phrases that you will see a lot in business or formal communication.