[ci skip] Add style guide about comma

guides/source/api_documentation_guidelines.md

@@ -84,6 +84,11 @@ English
 
 Please use American English (*color*, *center*, *modularize*, etc). See [a list of American and British English spelling differences here](http://en.wikipedia.org/wiki/American_and_British_English_spelling_differences).
 
+Comma
+-------
+
+Please use Oxford comma (*red, white, and blue* style). See [the detail of Oxford comma](http://en.wikipedia.org/wiki/Serial_comma).
+
 Example Code
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