Added a section to the README about projects using this CoC

We decided to use the Contributor Covenant CoC for Angular. :confetti_ball: This PR adds a short section to the README regarding projects using this CoC.

Because we have so many repos, I think the most pragmatic way to manage the CoC is to [maintain a fork here on the Angular org](https://github.com/angular/code-of-conduct) and link to `CODE_OF_CONDUCT.md` from the `README.md` of each project. In my fork, I edited the `README.md` to explain how the CoC document fits in with the rest of our contributing documentation, but included attribution for this project (please let me know if there's a better way for me to credit you).

Thanks! :)