Update CONTRIBUTING.md

Author: yangshun

Diff for: CONTRIBUTING.md

@@ -56,6 +56,8 @@ git push origin my-fix
 
 ## Code of Conduct
 
+Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please [read the full text](https://code.facebook.com/pages/876921332402685/open-source-code-of-conduct) so that you can understand what actions will and will not be tolerated.
+
 As contributors and maintainers of this project, we pledge to respect all people who contribute through reporting issues, posting feature requests, updating documentation, submitting pull requests or patches, and other activities.
 
 We are committed to making participation in this project a harassment-free experience for everyone, regardless of level of experience, gender, gender identity and expression, sexual orientation, disability, personal appearance, body size, race, age, or religion.