Document #17: Use "request" data for default app_url. [ci skip]

Author: polarlights

README.md

@@ -45,6 +45,14 @@ Create an initializer (e.g. `./config/initializers/swagger.rb`) and specify the
 ```ruby
 GrapeSwaggerRails.options.url      = '/swagger_doc.json'
 GrapeSwaggerRails.options.app_url  = 'http://swagger.wordnik.com'
+``````
+
+You can dynamically set `app_url` for each request use a `before_filter_proc`:
+
+```ruby
+GrapeSwaggerRails.options.before_filter_proc = proc {
+  GrapeSwaggerRails.options.app_url = request.protocol + request.host_with_port
+}
 ```
 
 You can set the app name, default is "Swagger".