Add available ivars to the README example

Author: tmm1

README

@@ -18,6 +18,19 @@
     include EM::HttpServer
 
     def process_http_request
+
+      # the http request details are available via the following instance variables:
+      #   @http_protocol
+      #   @http_request_method
+      #   @http_cookie
+      #   @http_if_none_match
+      #   @http_content_type
+      #   @http_path_info
+      #   @http_request_uri
+      #   @http_query_string
+      #   @http_post_content
+      #   @http_headers
+
       response = EM::DelegatedHttpResponse.new(self)
       response.status = 200
       response.content_type 'text/html'