The specification is available at https://gocodebox.github.io/lifterlms-rest/. Please note that this is a specification proposal which is not yet functional. The target release date for the REST API is August 15, 2019. The specification is subject to change prior to the official release of the REST API.
If you are planning on using the REST API (or connect a LifterLMS site to anything via Zapier) your early feedback will help us ensure that everything you're looking to do with the REST API is accounted for during the design phase.