Mention Q_INIT_RESOURCE in the plugin how-to.

Change-Id: I165f2f62693152a2ef94109a983dfd2513b58467
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>