The feature isn't available yet, and there's no official word on when it will be. However, it would make another useful tool in WhatsApp's arsenal against the spreading of misinformation. India, for example, has been particularly affected by malicious message forwarding, which led to WhatsApp imposing anti-forwarding limits, as well as forwarding notifications. Adding a quick and easy reverse image search feature like this will also help to stem the tide of fake news.