README: Add quip on keys.conf supporting env vars

Since Change-Id: If4f169d9ed4f37b6ebd062508de058f3baeafead
the insert_keys.py tool has had support for expanding
environment variable strings. This change addresses the lack
of an updated README covering said change.

Change-Id: I88e81ea58fb84110da3fc3cfb8b49fd0d6c027c2