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

README

@@ -114,7 +114,9 @@ mac_permissions.xml:
         any string specified in TARGET_BUILD_VARIANT. All tags are matched verbatim
         and all options are matched lowercase. The options are "tolowered" automatically
         for the user, it is convention to specify tags and options in all uppercase
-        and tags start with @.
+        and tags start with @. The option arguments can also use environment variables
+        via the familiar $VARIABLE syntax. This is often useful for setting a location
+        to ones release keys.
 
         NOTE: The pem files are base64 encoded and PackageManagerService, mac_permissions.xml
               and setool all use base16 encodings.