[FEEDBACK PLEASE] First PR posted for privacy & security explainer

I've posted a start to the privacy & security explainer
<https://github.com/immersive-web/privacy-and-security/pull/13>. This PR
addresses #2
<https://github.com/immersive-web/privacy-and-security/issues/2> (structure
of the explainer) and #4
<https://github.com/immersive-web/privacy-and-security/issues/4> (site
access to real-world geometry data).

For those interested in this topic, can you please provide feedback in the
next couple of weeks? I'll update with additional topics, but I'd like
feedback first to make sure the doc is heading in the right direction.

I won't email this thread with subsequent updates; if you're interested in
that please enable notifications for the PR.

Thanks everyone!
John

PR: https://github.com/immersive-web/privacy-and-security/pull/13