Updating contributing page to include code of conduct

Summary:
Add Code of Conduct

In the past Facebook didn't promote including a Code of Conduct when creating new projects, and many projects skipped this important document. Let's fix it. :)

#129

Reviewed By: SergioGuerreroFB

Differential Revision: D38267417

fbshipit-source-id: 0ac5c4b883c74b9967837b0946ba6939bd0c8771

CONTRIBUTING.mdown

@@ -1,5 +1,7 @@
 Facebook welcomes contributions to our SDKs.
 
+Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please [read the full text](https://code.facebook.com/codeofconduct) so that you can understand what actions will and will not be tolerated.
+
 All contributors must sign a CLA (contributor license agreement) here:
 
   https://code.facebook.com/cla/