Difference to base/upstream project

[hoijui]

The README mentions, that this project is based on https://github.com/Keats/jsonwebtoken.
I think it would be useful to mention why it was forked, and what the differences are.
I saw that https://github.com/Keats/jsonwebtoken has much more recent commits,
so my rough guess is that the reason to fork was not inactivity.

If you explain here, I can also try to make a PR for the README.