This updates content in README.md and breaks out some sections into
separate files, to keep the length more manageable.

Change-Id: I236ae43cd7323af8a11086ffe6c56ddb81b67a0c
Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/3028486


Reviewed-by: Jordan Bayles <jophba@chromium.org>
Reviewed-by: Ryan Keane <rwkeane@google.com>
Commit-Queue: mark a. foltz <mfoltz@chromium.org>