[docs] Add documentation to set up redirect URIs

[joshuadeguzman]

In most cases, people will attempt follow what's added on the front page of the package. It would be great to provide some context at the bare minimum how to setup redirect URIs for OAuth2.

For example, someone will get redirected to this screen in the default browser:

Outlining this on the README page would help save time for other developers:

The package should assume they're using this package mainly for macOS and not haven't integrated iOS yet. So some prerequisite setup might not have been made for the latter.