Subject: Add the Google's Code Review Developer Guide as a reference in our code review guide?


Hi, all,

Here, I am proposing to add the Google's Code Review Developer Guide
<https://google.github.io/eng-practices/review/> as a reference in our code
review guide. The guide looks very reasonable to our Spark development too.
We do not need to completely follow each rule but it is a good guide in
general.

Below is my PR: https://github.com/apache/spark-website/pull/226/
<https://github.com/apache/spark-website/pull/226/files> to make a change
in our Spark's website.

Cheers,

Xiao