document using ES6 with Middleman 4 and Sprockets 3.x

Author: vast

README.md

@@ -1,8 +1,38 @@
 # Middleman-ES6 [![Build Status](https://semaphoreci.com/api/v1/vast/middleman-es6/branches/master/shields_badge.svg)](https://semaphoreci.com/vast/middleman-es6)
 
-Middleman (Sprockets) extension adding support for ES6 (via babel-transpiler).
+Middleman _3_ extension adding support for ES6 (via babel-transpiler).
 
-## Installation & Usage
+## Middleman 4
+
+If you use Middleman 4, then there is no need in `middleman-es6` extension. To add ES6 support
+to Middleman 4 add `middleman-sprockets`, `sprockets-es6` to Gemfile and configure it to handle
+assets with `.es6` extension:
+
+```ruby
+# Gemfile
+
+gem 'middleman-sprockets', '~> 4.0.0.rc'
+gem 'sprockets-es6'
+```
+
+```ruby
+# config.rb
+
+require 'sprockets/es6'
+activate :sprockets do |s|
+  s.supported_output_extensions << '.es6'
+end
+```
+
+If you need Babel's polyfill:
+
+```javascript
+// application.js.es6
+
+//= require babel/polyfill
+```
+
+## Installation & Usage with Middleman 3
 
 Add `middleman-es6` gem to your application's Gemfile: