Add note for default writing to os.Stderr (rs#97)

README.md

@@ -60,7 +60,7 @@ func main() {
 
 // Output: {"time":1516134303,"level":"debug","message":"hello world"}
 ```
-
+> Note: By default log writes to `os.Stderr`  
 > Note: The default log level for `log.Print` is *debug*
 
 ### Contextual Logging