How to contact a user who wrote a review to my app?
Since a few days, you now have the ability to answer to reviews, right from iTunes Connect.

Go to My Apps > YourApp > Activity > Reviews and you will be able to click on Answer for each review. 

Your answers will be available directly from the App Store for people running on iOS 10.3, and I think the reviewer also receive a mail or notification that you answered.

It a huge step from Apple to finally offer this !