Add Google JavaScript Style Guide

Added a few sentences about following Google's JavaScript Style
Guide.

Change-Id: Ie96c19daf569ca451a805820b856140bf072f6a4
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index f59fe9f..798bc05 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -85,6 +85,9 @@
 
 ### JavaScript
 
+We follow the
+[Google JavaScript Style Guide](https://google.github.io/styleguide/jsguide.html).
+
 A .clang-format file that closely approximates our JavaScript coding style is:
 ```
 ---