readme: add example of context logging

README.md

@@ -108,6 +108,16 @@ func main() {
     "omg":    true,
     "number": 100,
   }).Fatal("The ice breaks!")
+
+  # A common pattern is to re-use fields between logging statements by re-using
+  # the logrus.Entry returned from WithFields()
+  contextLogger := log.WithFields(log.Fields{
+    "common": "this is a common field",
+    "other": "I also should be logged always",
+  })
+
+  contextLogger.Info("I'll be logged with common and other field")
+  contextLogger.Info("Me too")
 }
 ```