Include link to original source of repo

[edunham]

This repo is often used in examples, where it's copied wholesale or inserted via submodule into another repository.

If a user wants to modify the JS app's actual source, rather than just the local copy, they will benefit from having the link.

I discovered this omission when reviewing an example repo that uses this app, https://github.com/noinarisak/okta-terraform-hub-spoke-oidc, and I had to go ask the author about where the React sample came from rather than just seeing it in the readme.