readme: update with example of logging to file

README.md

@@ -87,7 +87,8 @@ func init() {
   // Log as JSON instead of the default ASCII formatter.
   log.SetFormatter(&log.JSONFormatter{})
 
-  // Output to stdout instead of the default stderr, could also be a file.
+  // Output to stdout instead of the default stderr
+  // Can be any io.Writer, see below for File example
   log.SetOutput(os.Stdout)
 
   // Only log the warning severity or above.
@@ -138,7 +139,15 @@ var log = logrus.New()
 func main() {
   // The API for setting attributes is a little different than the package level
   // exported logger. See Godoc.
-  log.Out = os.Stderr
+  log.Out = os.Stdout
+
+  // You could set this to any `io.Writer` such as a file
+  // file, err := os.OpenFile("logrus.log", os.O_CREATE|os.O_WRONLY, 0666)
+  // if err == nil {
+  //  log.Out = file
+  // } else {
+  //  log.Info("Failed to log to file, using default stderr")
+  // }
 
   log.WithFields(logrus.Fields{
     "animal": "walrus",