Add Screen interface integration info.

[VergeA]

The decision has been made to not fence Screen interface properties within fenced frames, see doc for rationale. This PR documents that decision explicitly.

[domfarolino]

@VergeA it looks like the IPR bot is unhappy. Can you follow the instructions in https://labs.w3.org/repo-manager/pr/id/WICG/fenced-frame/121 and create a W3C account and link it here etc... Let me or cwilso@ if you have any trouble.

[VergeA]

Hey @domfarolino, I went through the whole process yesterday (made a W3C account, linked it here, requested nomination from advisory committee and had it granted). Got an email saying:

"Andrew Verge became a participant in the
Web Platform Incubator Community Group, representing Google LLC.
This person was nominated by their Advisory Committee Representatives."

So not entirely sure why the bot is still grumpy

[VergeA]

Ah, it appears to have righted itself after I linked the ipr bot site to my Github account as well. Everything looks green now.