#Gemfile
gem 'rswag-api'
gem 'rswag-ui'

group :development, :test do
  gem 'rspec-rails',         '~> 3.8.1'
  gem 'rspec-rails-swagger', '~> 0.1.5'
...
end

#../config/routes.rb

Rails.application.routes.draw do
  mount Rswag::Ui::Engine => '/api-docs'
  mount Rswag::Api::Engine => '/api-docs'
...#other routes come here
end