Link to COC from CONTRIBUTING.md

We are about to add a `CODE_OF_CONDUCT.md` doc and it makes sense to surface it here.

Author: flarnie

CONTRIBUTING.md

@@ -5,6 +5,9 @@ We love to have your help to make php-webdriver better!
 Feel free to open an [issue](https://github.com/facebook/php-webdriver/issues) if you run into any problem, or
 send a pull request (see bellow) with your contribution.
 
+## Code of Conduct
+The code of conduct is described in [`CODE_OF_CONDUCT.md`](CODE_OF_CONDUCT.md)
+
 ## Workflow when contributing a patch
 
 1. Fork the project on GitHub