add events code walkthrough video link (facebook#7633)

I think if we do another one of these it'd be good to turn this into a list, but this is good for now.

Thanks @spicyj!
cc @gaearon

CONTRIBUTING.md

@@ -32,7 +32,7 @@ Just make sure to run the whole test suite before submitting a pull request!
 
 **Working on your first Pull Request?** You can learn how from this *free* series [How to Contribute to an Open Source Project on GitHub](https://egghead.io/series/how-to-contribute-to-an-open-source-project-on-github)
 
-You may also be interested in watching [this short video](https://www.youtube.com/watch?v=wUpPsEcGsg8) (26 mins) which gives an introduction on how to contribute to the React JS project.
+You may also be interested in watching [this short video](https://www.youtube.com/watch?v=wUpPsEcGsg8) (26 mins) which gives an introduction on how to contribute to the React JS project. As well as [this one](https://www.youtube.com/watch?v=dRo_egw7tBc) (66 mins) which gives a code walkthrough of the event system.
 
 The core team will be monitoring for pull requests. When we get one, we'll run some Facebook-specific integration tests on it first. From here, we'll need to get another person to sign off on the changes and then merge the pull request. For API changes we may need to fix internal uses, which could cause some delay. We'll do our best to provide updates and feedback throughout the process.