entry: break out time, level and message from data

README.md

@@ -261,6 +261,9 @@ type MyJSONFormatter struct {
 log.Formatter = new(MyJSONFormatter)
 
 func (f *JSONFormatter) Format(entry *Entry) ([]byte, error) {
+  // Note this doesn't include Time, Level and Message which are available on
+  // the Entry. Consult `godoc` on information about those fields or read the
+  // source of the official loggers.
   serialized, err := json.Marshal(entry.Data)
     if err != nil {
       return nil, fmt.Errorf("Failed to marshal fields to JSON, %v", err)