Add Ruby on Rails book "Api on Rails 6" (#4777)

free-programming-books.md

@@ -2541,6 +2541,7 @@ Kerridge (PDF) (email address *requested*, not required)
 #### Ruby on Rails
 
 * [A community-driven Rails style guide](https://github.com/bbatsov/rails-style-guide)
+* [Api on Rails 6](https://github.com/madeindjs/api_on_rails) - Alexandre Rousseau
 * [Building REST APIs with Rails](http://apionrails.icalialabs.com/book)
 * [Kestrels, Quirky Birds, and Hopeless Egocentricity](https://leanpub.com/combinators/read) - Reg Braithwaite
 * [Learn Ruby on Rails as You Modify a Craigslist Clone](http://www.thinkful.com/learn/ruby-on-rails-tutorial/)