Multi authentication script doc

[mzico]

This is our first step of combining two custom authN script. I think we can put it there in our public doc for users to see how such scenario works and how it's possible to 'combine' two scripts together.

In this situation we combined CAS2 script + Duo script and generated a combined script which allowing users to:

• 1st authentication with organizational remote CAS2 server
• 2nd phase of authentication with organization's Gluu Server's Duo

Here is the doc: https://github.com/GluuFederation/oxAuth/tree/master/Server/integrations/cas2_duo