By default the Wordpress JSON API: WP-API doesn't include Advanced Custom Fields. This plugin fixes that.
Grown from https://gist.github.com/rileypaulsen/9b4505cdd0ac88d5ef51 but a plugin is nicer: I didn't want to modify the other plugin
Not actively in development, as I'm no longer involved with the project this was developed for. If you want to take over, let me know! PanMan