Add Code of Conduct to the contributing guide

CONTRIBUTING.md

@@ -45,6 +45,11 @@ then you'll need to sign a [corporate CLA][corpcla].
 You can sign these electronically (just scroll to the bottom). After that,
 we'll be able to accept your pull requests.
 
+## Code of Conduct
+
+Please note that this project is released with a Contributor Code of Conduct. By participating in this project you agree to abide by its terms. See [Code of Conduct][code-of-conduct] for more information.
+
 [gcloudcli]: https://developers.google.com/cloud/sdk/gcloud/
 [indvcla]: https://developers.google.com/open-source/cla/individual
 [corpcla]: https://developers.google.com/open-source/cla/corporate
+[code-of-conduct]:https://github.com/GoogleCloudPlatform/gcloud-java/blob/master/CODE_OF_CONDUCT.md