Add a fork link back to upstream

[elliottkember]

The repo is currently accidentally a clone and not a fork. We should create a new repo as a fork!

[baldurh]

I believe the best way to achieve this is to do the following:

• Merge all outstanding pull requests
• Fork the upstream (https://github.com/ctrlplusb/react-universally)
• Force push all branches from this repo to the newly forked repo
• Delete this repo
• Rename the new repo