[Development] Q_PLUGIN_METADATA documentation

[Lorn Potter]

Hi *,

While working on QTBUG-31900, there was a change in qtbase - 732dcfe7
which broke loading of sensorgesture plugins.

It turns out, the plugin loader is now only loading the first 
sensorgesture plugin found.

If I remove the FILE portion of the Q_PLUGIN_METADATA entry they all 
load and tests pass.
The json file for all sensorgestures plugins contains the same thing:
  { "Keys": [ "notused" ] }
This has been working fine since 5.0

The documentation here:
http://qt-project.org/doc/qt-5.0/qtcore/qtplugin.html#Q_PLUGIN_METADATA

says nothing about the contents of the json file, what is possible for 
it to contain, what it should contain, or even it's purpose.

The example mentioned there only refers back to this page.

Can someone explain this? and perhaps beef up that documentation?

-- 
Lorn Potter
Senior Software Engineer, QtSensors/QtSensorGestures/QtSystemInfo