Add metadata command/key to README.md of gopts plugin
This is an addendum to
- infos/recommends =
- infos/placements = procgetstorage
- infos/status = recommended productive maintained nodep libc
-- infos/metadata = args args/index command opt opt/long opt/arg opt/flagvalue opt/help opt/hidden opt/# opt/#/long opt/#/arg opt/#/flagvalue opt/#/hidden env env/#
+- infos/metadata = args args/index command command/key opt opt/long opt/arg opt/flagvalue opt/help opt/hidden opt/# opt/#/long opt/#/arg opt/#/flagvalue opt/#/hidden env env/#
- infos/description = Parses command-line options using elektra-opts
## Introduction
