In the past Apple has not provided any functionality for developers to respond to App Store reviews, but as of iOS 10.3 that has changed. Apple now allows developers to finally post public responses to their app reviews.
If you're interested in responding to a review, launch iTunes Connect and navigate to My Apps > Ratings & Reviews. Find a review that needs a response, type your message, and hit Submit. If necessary, you can edit your response after you have posted it.
Developers interested in receiving notifications for new app reviews should sign up for a free trial at Ratings Catcher. You can also access our customizable review response templates and see all of your new iOS/Mac/Google Play reviews.