Update README for html_embedded_css_file

Author: oestrich

README.md

@@ -139,6 +139,10 @@ RspecApiDocumentation.configure do |config|
   # Change how the post body is formatted by default, you can still override by `raw_post`
   # Can be :json, :xml, or a proc that will be passed the params
   config.post_body_formatter = Proc.new { |params| params }
+
+  # Change the embedded style for HTML output. This file will not be processed by
+  # RspecApiDocumentation and should be plain CSS.
+  config.html_embedded_css_file = nil
 end
 ```