Create vlog_guide.md

[tonyyang-svail]

This is a follow up for switching logging to VLOG. I feel we need a VLOG level guide for the PaddlePaddle developers. Here is what I have in mind.

[helinwang]

Maybe add a small intro for what is VLOG, such as (taken from the link you provided):

Verbose Logging

When you are chasing difficult bugs, thorough log messages are very useful. However, you may want to ignore too verbose messages in usual development. For such verbose logging, glog provides the VLOG macro, which allows you to define your own numeric logging levels.

[helinwang]

I think it's important to use the vmodule flag:

vmodule (string, default="")

Per-module verbose level.

--vmodule=mapreduce=2,file=1,gfs*=3 --v=0

[wangkuiyi]

We are going to follow the convention adopted by Chromium to use VLOG instead of LOG(INFO) to generate debug information. Please be aware that usually log messages are still via LOG.

For more information about VLOG, please refer to the official document of glog.

VLOG requires a verbose level parameter, like that LOG requires a severity level. This document explains the verbose level value for various kinds of VLOG messages.

[tonyyang-svail]

VLOG guide has been added to CONTRIBUTING.md.