Store metadata in a PostgreSQL database and manage it from QGIS.
PgMetadata is released under GNU General Public License v2.0 or any later version.
Presentation, user guide, installation, Lizmap Web Client, everything is there.
HeatherHillers/qgis-pgmetadata-plugin
QGIS Plugin to manage some metadata from PostgreSQL layer
PLpgSQLGPL-2.0