Link to key PRs and meeting notes

Would love if you could link/reference some of the core React teams's activity around fiber into this document. It'd make it easier to understand the actual implementation and current status.

You probably know this better, but I'm seeing the following items:
- https://github.com/reactjs/core-notes/blob/master/2016-06/june-23.md#update-on-fiber
- https://github.com/facebook/react/issues/6170
- https://github.com/facebook/react/pull/6690
- https://github.com/facebook/react/pulls?utf8=%E2%9C%93&q=is%3Apr%20%5Bfiber%5D

Cheers and thanks for putting this together!