Doc: Added information about classname in qmldir

QML modules that depend on C++ plugins must provide
the plugin class name information in their qmldir
to support statically built Qt Quick applications.

Task-number: QTBUG-35369
Change-Id: I15432fec4f8a72be4f851da061ac0c091696d8c9
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>