Add documentation

README.md

@@ -61,6 +61,38 @@ You can to file and STDOUT at the same time, you just need to assign new logger
     log_file = File.open('path/to/your/logfile.log', 'a')
     log_file.sync = true
     logger Logger.new GrapeLogging::MultiIO.new(STDOUT, log_file)
+    
+### Logging via Rails instrumentation
+
+You can choose to not pass the logger to ```grape_logging``` but instead send logs to Rails instrumentation in order to let Rails and its configured Logger do the log job, for example.
+First, config ```grape_logging```, like that:
+
+    class MyAPI < Grape::API
+      use GrapeLogging::Middleware::RequestLogger,
+        instrumentation_key: 'grape_key',
+        include: [ GrapeLogging::Loggers::Response.new,
+                   GrapeLogging::Loggers::DatabaseTime.new,
+                   GrapeLogging::Loggers::FilterParameters.new ]
+    end
+    
+and then add an initializer in your Rails project:
+    
+    # config/initializers/instrumentation.rb
+    
+    # Subscribe to grape request and log with Rails.logger
+    ActiveSupport::Notifications.subscribe('grape') do |name, starts, ends, notification_id, payload|
+      Rails.logger.info payload
+    end
+   
+The idea come from here: https://gist.github.com/teamon/e8ae16ffb0cb447e5b49
+
+There's some advantage to use this method:   
+
+  - You could use a logger that does not implement the ```formatter=```.   
+  Defaults Rails 3 (ActiveSupport::BufferedLogger) does not implement it.   
+  The Logging gem (https://github.com/TwP/logging) does not implement it neither
+  
+  - If you use a logger that already format logs (as the Logging gem), the logs will be formatted by your logger
 
 ### Logging exceptions