Per suggestion from @tvdijen

Add some basic documentation on how the new Twig templating system works.

It was already added under 1.15

Per https://groups.google.com/d/msg/simplesamlphp/ATKQksysR5s/3VhjdLbTAgAJ
there is a feature to allow IdPs to respond to requests sent to different
IdP entityIDs.

Google will create an entityId with your domain, as described in #9.

Might not be complete, needs check with current G Suite interfaces.
But improves on the current situation.

It can now be used with \SimpleSAML\Auth\Simple, although the old name still works too.