Updated README

- Change to use Markdown
- Updated use call to insert_before for Rails configuration

Fixes #32

README.md

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+# Rack CORS Middleware
+
+`Rack::Cors` provides support for Cross-Origin Resource Sharing (CORS) for Rack compatible web applications.  The CORS spec allows web applications to make cross domain AJAX calls without
+using workarounds such as JSONP.  For a thorough write up on CORS, see this blog post:
+
+http://www.nczonline.net/blog/2010/05/25/cross-domain-ajax-with-cross-origin-resource-sharing/
+
+Or for all the gory details, you can read the spec here:
+
+http://www.w3.org/TR/cors/
+
+## Installation
+
+Install the gem:
+
+`gem install rack-cors`
+
+Or in your Gemfile:
+
+```ruby
+gem 'rack-cors', :require => 'rack/cors'
+```
+
+
+## Configuration
+
+### Rack
+
+You configure `Rack::Cors` by passing a block to the `use` command:
+
+```ruby
+use Rack::Cors do
+  allow do
+    origins 'localhost:3000', '127.0.0.1:3000',
+            /http:\/\/192\.168\.0\.\d{1,3}(:\d+)?/
+            # regular expressions can be used here
+
+    resource '/file/list_all/', :headers => 'x-domain-token'
+    resource '/file/at/*',
+        :methods => [:get, :post, :put, :delete, :options],
+        :headers => 'x-domain-token',
+        :expose  => ['Some-Custom-Response-Header'],
+        :max_age => 600
+        # headers to expose
+  end
+
+  allow do
+    origins '*'
+    resource '/public/*', :headers => :any, :methods => :get
+  end
+end
+```
+
+### Rails
+Put something like the code below in `config/application.rb` of your Rails application. For example, this will allow GET, POST or OPTIONS requests from any origin on any resource.
+
+```ruby
+module YourApp
+  class Application < Rails::Application
+
+    # ...
+
+    config.middleware.insert_before "ActionDispatch::Static", "Rack::Cors" do
+      allow do
+        origins '*'
+        resource '*', :headers => :any, :methods => [:get, :post, :options]
+      end
+    end
+
+  end
+end
+```
+
+Refer to `examples/rails3` and `examples/rails4` for more details.
+
+See http://guides.rubyonrails.org/rails_on_rack.html for more details on rack middlewares or
+http://railscasts.com/episodes/151-rack-middleware.