According to NDTV and some other sources, the Facebook-owned application already began rolling the feature out to the general public, so it's possible that you can already use it. To find out if you have access to it, simply press on the message you want to respond to for a few seconds until the action bubble/bar pops up. Next to star, trash, copy and forward is a new option that looks like the arrow typically associated with reply. Click that, and the message you want to quote will show up right above your text box like in the screenshots below. If you can't seem to make it work, try reinstalling your app or checking again in a day or two -- you might have it then.