Describe in more detail how to initialize the ad container

[pietermees]

1. Player loads cross domain page, passing in 3rd party js script as cgi parameter or through postmessage.
2. The cross domain page will then take that parameter and load ad the script as a tag. Keep in mind, the publisher controls both the original page as well as the cross domain frame.

[ryanthompson591]

Throughout the doc we should also consistently use 1 definition. secure iframe = cross domain iframe = sandboxed iframe

[ryanthompson591]

Here is the current description: https://github.com/InteractiveAdvertisingBureau/SIVIC/issues/84