Mention fallback code owners in setup guide
Mention fallback code owners in setup guide

Signed-off-by: Edwin Kempin <d43fb6c98c97e553b671ed9a9318100f1de386e7@google.com>
Change-Id: I4190858898b43a0a6e2b97ea3672adf8bccd913e

GerritCodeReview/plugins_code-owners,GerritCodeReview/plugins_code-owners,GerritCodeReview/plugins_code-owners,GerritCodeReview/plugins_code-owners