By default the Wordpress JSON API: WP-API doesn't include Advanced Custom Fields. This plugin fixes that.
This plugin was written for the now depricated v1 of the WP JSON API. This updated version works with the new v2 WP REST API
Grown from https://gist.github.com/rileypaulsen/9b4505cdd0ac88d5ef51 but a plugin is nicer: I didn't want to modify the other plugin