add usage description for a standalone javascript file

README.md

@@ -20,10 +20,18 @@ Or install it yourself as:
 
 ## Usage
 
-Add to your `app/assets/stylesheets/application.js`
+Add this line to your `app/assets/stylesheets/application.js` to compile as a part of application.js
 
     //= require modernizr
 
+Or add this line to your `config/environments/production.rb` to compile as a standalone javascript file
+
+    config.assets.precompile += %w( modernizr.js )
+
+And include it in your layout like `app/views/layouts/application.html.erb`
+
+    <%= javascript_include_tag 'modernizr' %>
+
 ## Contributing
 
 1. Fork it