Add a few more code-pen examples of working code

[CaptRat]

Instead of (only) the final version, it would be really helpful for us newbies if you added more working code samples throughout. Once mine broke, I couldn't figure how to continue moving forward - didn't understand if I put changes in the correct places and didn't know where the code isn't supposed to work.

Before submitting a pull request, please make sure the following is done:

1. Fork the repository and create your branch from master.
2. If you've added code that should be tested, add tests!
3. If you've changed APIs, update the documentation.
4. Ensure the test suite passes (npm test).
5. Make sure your code lints (npm run lint).
6. Format your code with prettier (npm run prettier).
7. Run the Flow typechecks (npm run flow).
8. If you added or removed any tests, run ./scripts/fiber/record-tests before submitting the pull request, and commit the resulting changes.
9. If you haven't already, complete the CLA.

[facebook-github-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. In order for us to review and merge your code, please sign up at https://code.facebook.com/cla - and if you have received this in error or have any questions, please drop us a line at cla@fb.com. Thanks!

If you are contributing on behalf of someone else (eg your employer): the individual CLA is not sufficient - use https://developers.facebook.com/opensource/cla?type=company instead. Contact cla@fb.com if you have any questions.

[gaearon]

cc @brigand, something you could look at?

[brigand]

@gaearon yep.

[gaearon]

This will be addressed by #9454.
Thanks for raising it.