User registration using Drupal 8's REST API has been in flux but appears to be stabilized (for now).
There are many blog posts on using Drupal 8's REST API. Getting a user registered using REST, however, seems to be in flux. This post will get you set up for using the REST API, and in particular, will outline the steps to get a new user registered on a Drupal 8.3.x site.
First, enable the 4 core Web Services modules: HAL, HTTP Basic Authentication, RESTful Web Services (REST), and Serialization.