Add link to COC from CONTRIBUTING.md

We are about to add a `CODE_OF_CONDUCT.md` and it makes sense to link to it from here.

Author: flarnie

CONTRIBUTING.md

@@ -5,6 +5,9 @@ Contributions are **welcome** and will be fully **credited**.
 
 We accept contributions via Pull Requests on [Github](https://github.com/facebook/facebook-php-sdk-v4).
 
+## Code of Conduct
+The code of conduct is described in [`CODE_OF_CONDUCT.md`](CODE_OF_CONDUCT.md)
+
 
 ## Pull Requests